We need to be very careful in our terminology, here.  I assume 51 and 31 are the boats off to the left of our picture, at 18 seconds into the clip.  At that point, they are on opposite tacks, both sailing above 90 degrees to the true wind, and rule 18 does not apply.  So neither boat can be "clear ahead" (see the last sentence of the definition Clear Astern, Clear Ahead; Overlapped).  If you mean, it looks as if 51 could cross 31 on port tack, how can you tell?  Video is so notoriously bad in the line of sight as to be essentially irrelevant.  If 31 tacked and claimed that she did so to avoid contact with 51, I think she'll win that in the protest room.  For a similar situation, see World Sailing Case 50.  

Looked like from the limited view and quality of the video that 31 had a reasonable apprehension of a collision...

Not sure we're talking about the same incident, as the starboard tack boat in the first incident with 51 is 35.  31 may be the black boat that crosses clear ahead of 51 on port at the pin end. No incident there. 
'At the 18 second mark...' is irrelevant to the incident between 51 and 35. The incident happens after the start and appears to be just beyond the start line, 51 and 35 (dark blue hull, yellow deck) are on opposite tacks so Rule 10 and, as Rob says, Case 50 apply.  As does E4.3 (b).  This is where my knowledge runs out a bit. E4.3 (b) says one turn penalties until her advantage is lost.  Given that 35 is also further disadvantaged by 15, when she breaks rule 10 and 35 has, at that point, made no ground since the incident with 51, does 51 have to complete one-turn penalties until 35 has sailed ahead of her on the race course?  And what of 15? She would only have to complete turns until 35 is ahead of her, giving her a significant advantage over 51.  But, as I'm often reminded, the rules may not be fair, but they are the rules...

Both red boats DSQ'd. Dark hull with lime green deck tacked to avoid collision with red port tack boat that broke RRS 10, which sailed on and didn't take any penalty. Again, the dark hull and lime deck boat was fouled by red hull 51 RRS 10 and 14.

Thanks to all feedback. I guess the onus of proof is on the port tack boat and would need plenty of evidence to avoid a DSQ.

Phil, there were two fouls; the second one you didn't mention. The second red boat #15 also fouled the lime green decked boat. Although it was dead in the water from the first foul, it was back on starboard when 15 tacked into it its port quarter. 15 broke either 13 While Tacking, or 10 if it had completed its tack, plus 14 for the contact.
Edited: changed #51 to #15 for the second foul.

In the video, prior to 35 changing course, it appears as though 51 has room to pass ahead of 35.  It seems that 35 is attempting to hunt 51 or tack in order to assume her proper course rather than to avoid 51.   If 35 had to bear away to avoid 51, I would find that a far more convincing argument that 35 had a genuine apprehension of collision.  Based on this video alone, I would find it very hard to disqualify 51, especially when considering that the distance between the boats would actually be further than appears due to lens foreshortening.

Murray, one thing to remember with RC racing, is not everyone can see precisely, close crossing incidents due to the distance, size of boats, angles etc... so it places a greater burden on a port tack boat to be certain they can cross. Therefore if a starboard tacker thinks they won't, they're obliged to take avoiding action, or be guilty as well in breaking R14, when it looks inevitable. You can't read the mind of the skipper of the starboard tacker and say they're hunting (breaking 16). They could be standing on the shore with a different vantage point. The port tacker better have a really good time/distance argument that will be tough to prove. You also need to take into account the extra difficulty in boat handling in puffy conditions and chop, bearing away while getting a gust etc... 

Murry,

Don't forget, in breeze, all boats round up and tack easier than bearing away...

Phil B wrote:

Contrary to popular opinion, there is no such onus of proof on the port tack boat.  However, "P must present adequate evidence to establish either that S did not change course or that P would have safely crossed ahead of S
and that S had no need to take avoiding action."  (WS Case 50).

As I said, based on this video alone (as it is the only evidence we have for this case), I would be very reluctant to disqualify 51.  The apprehension on the part of S must be both genuine and reasonable.  Given the 2-3 boat lengths between 51 and 35 at the time 51 started to cross 35, and the separation between the boats, as long as 35 held her course toward the mark, 51 would cross ahead.  But, when 35 changes course above the mark, she is obligated to give 51 room to keep clear. 

Tim, just as one cannot read the mind of the skipper, one cannot surmise where they are standing.  The video is the only evidence we have.

Murray,
Nice illustrations, however, we're asked for an opinion and you have concluded that 35 hunted, based on video evidence. Although the video "evidence" is nice to see, in a real hearing, the vantage point of the skippers would be taken into account, more than the vantage point of the videographer. Based on that reality, I'd conclude that if 35 felt they had to tack, given the variable conditions they were sailing in, that 51 didn't keep clear. The definition of Keep Clear a) says a boat is keeping clear if the ROW boat can sail their course with no need to take avoiding action. In the absence of 51, 35's course could be changing frequently on a beat, following wind shifts, gusty conditions, responding to sea state etc.. Rule 16 shouldn't be used as a shield for a give way boat taking liberties with respect to their first obligation of Keeping Clear of a ROW boat. I understand that one of the intents of 16 is to reduce collisions, and stop unsportsman-like hunting where the give way boat can't give way, but it gets abused by port tackers too frequently e.g. boats on starboard rounding a windward mark turn downwind on their proper course only to find a port tacker in their path, still beating to the mark inside the zone, claiming the starboard tackers have to keep clear because they have changed their course by rounding the mark !! The port tacker has first and foremost, put themselves in that position and is required to keep clear.

Tim,
I don't remember saying that I concluded 35 hunted 51.  Yes, I agree that the vantage point of the skippers would be taken into account.  However, you are basing your assumptions on information that is not available.  The video and the illustrations above show that 35 could have sailed her course with no need to take avoiding action, up until the point where 35 changed her course (as shown in the last illustration).  When 35 changed her course, 16.1 requires her to give 51 room to keep clear, which she does by luffing.  We have no information as to the reason/s for 35 changing course when she did.  You are surmising that 35 changed course to avoid 51.  It may have been 25 was hunting 51.  Or it may have been that 35 was just wanting to tack but turned too slowly in the conditions to complete the tack and ended up in irons.  To that point, and to answer Phil's question, there is insufficient evidence with the video alone to determine if 51 broke a rule and there is no onus on 51 to prove that she didn't.

In your example of abuse, the starboard tack boat, having right of way, does not have to keep clear of the port tack boat.  However, when the starboard tack boat changes course while rounding the mark, she is required to give the port tack boat room to keep clear (rule 16.1).  That is plain and simple, not an abuse by the port tack boat.

Murray " It seems that 35 is attempting to hunt 51" extract from your earlier comment.

Tim,
seems - "give the impression or sensation of being something or having a particular quality."  
Nothing conclusive about that. Especially when you actually read the whole sentence which presents two possible alternative reasons for S changing course other than just to avoid P.

The action video clearly shows the opposite of what Murray's selective screenshots suggest.