Having thought about it for a bit, I think that Bos caught his brake lever in behind Impey's bar end as he was going past, and that the hand on the back was a useless attempt to ward off the impending crash.
Impey was moving left, and I think Bos had spotted a big enough gap from 3 or 4 bike lengths back, put his head down and gone for it, only to find it had closed before he got there.
If Bos had grabbed the jersey and thrown Impey, it would have equally have thrown Bos to the right. If Bos had hit the barriers, he would have come down onto Impey and sent him the other way.