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).