I slo-mo'd it. Rider in yellow got his right side of bars behind rider in red's bars; made contact, rider in red slowed up a bit, pushing rider in yellow's bars into a quick right jerk he couldn't recover / prevent.
After all these years, I find myself sticking out my elbows like curb feelers whenever things get tight. And avoiding bar to bar contact.
"have fun and be kind"
- an internet post
It looks like the rider on the left was loosely following the rider in front of him. When the rider in front moved to the right, the rider tried to force his way over. When the rider on the left touched, the rider to his right nudged with his elbow twice. The 2nd time moved the rider's arm slightly aft. Counter-steering cause the bike to go down.
Moral of the story:
Protect yourself if you're if you're attempting to physically force someone off a wheel/
or simply drop back slightly and move into the opening without crashing (preferred method).
But it's pretty hard to tell for certain what happened based on the video, so there's not much sense in analyzing it to death.
Follow up question ... Say there is a group of 3-4 off the front with a reasonable gap. You bridge across solo and are hurting. What is the etiquette rule for skipping a couple of pulls to recover well enough to be a productive member of the break?
Inexperienced racers ***** about etiquette violations because they think etiquette is important and they want everyone to be polite. Experienced racers ***** about etiquette violations because it's a good way to browbeat an inexperienced racer into working too hard in the break.
Seriously, if you skip a couple pulls to recover and then start doing a normal rotation, you're unlikely to get any stick. Especially since if you can bridge more than about a 10 second gap solo, you're likely very strong and will be a big help to that break.
Etiquette is for those who think it is a training ride and everyone needs to pull the same amount at the same pace. For every pull you miss expect a heap of crap from someone in the group. But in general no one cares and might not even notice your first missed pull ... make sure you are still breathing hard. Then on the second missed pull you are likely to get someone to say something to you to encourage you, after that it can get nasty. Especially if you are on the big team and they are a bunch of solo riders.
Overall for me it comes down to how much do I want it to succeed and how much can I offer at their pace. They are not going to be happy with anyone sitting on if they were working well before you go there. And they will likely remember it for the next race, so be prepared to have people not work with you in race two if you screwed up their break in race one.
Me, I love to be the jerk and often I lack the fitness to be in the break in the first place and just want a sprint. so I slack off as much as possible, often killing the break and pissing people off. Then wait for the sprint or hope a teammate joins so that I can kill myself and not have to worry about sprinting. To me, my teammates matter much more than the riders in the break, I am just using them to improve my odds of winning. I could care less about their odds of winning ... its a race. But people will remember, so make sure you buy them a beer after the race if you screwed up their break, then they will love you and not care.
In my scenario, the teams were more or less limited to as many as two guys. In the break were two of the GC leaders for the stage race and the third was another strong guy. The break was about 12 seconds and I closed it to like 9 by jumping hard from about 5th wheel to not bring anyone up with me. When I was my closest to connecting, I was pretty fatigued, but was thinking a) will this stay away and b) if I connect will I be able to hang on? I convinced myself to regroup. That was a mistake as they stayed away to finish 1, 2, 3. I think if I sold out I could have made the connection, but would have had to sit on for a couple of pulls before doing any work. With that, once recovered, I would have been able to contribute and not just sit in. I'll remember for next time. Worst case is I connect, and then fall back off.
So for me the answer to the original question is: "ride with more experienced racers who will help you figure it out".
As a ball-sport lifer, I will admit to having strong legs but not a strong Race IQ.
Cat 2 O-Meter ..... about 1/3rd of the way there.