The video I am referencing is the one linked in that shows the zoomed in footage.
When the horses stop, the driver is shortening the lines and gathering contact. IF you look at his elbow, they are really far back so that makes me wonder if he just did not have enough contact before the halt. His feet are pushing against the dash and this hands are parallel with his bellybutton.
The horses stop and then his hands go forward almost to his knees to gather to make contact.
Then blurry fuzzy hand shaking footage. I watched the right swing pair (the head bobber) and he has a buxton bit in, so the line would not / should not catch on that, he also just looks to the right, not so much a bob. The third pair or second swing pair left horse bobs his head more than the other head bobber.
at the 11 second mark you see the driver collect the rail side horses to prepare to turn and again, his hands are at his belly button
Then at the 14-15 second mark, the co-driver is paying attention to the dog, and the driver realizes something is not right.
at 16 seconds, the co-driver notices something is wrong and the leaders are not backing into the swingle tree. (looks like the driver told him to dismount)
AT this point, the left rein may have snagged on the backstrap. but I still doubt it because the turn would have started here.
At 17 seconds he is gathering his lines some more and the leaders have already started to turn to the right into the rail and the right lead horse could look at the drive he is so far turned.
At 18 the co-driver hops off and the turn made by the leaders is passed the 90 degree turn and sees he has no where to go but in between swing pair two and three.
Right leader is dragging left leader with him the entire time.
@goodhors I understand how lines work. I was a four in hand groom for coaching and FEI CDEs for over a decade and I can drive a four. I have seen wheelers snag a leaders line, that was way more than a wheeler hanging on a line.
I do think the right leader took it upon himself to do the move on his own and realized he got stuck so went between pairs two and three. OR the driver did not have any control/contact from the halt.
IMHO.