[OHBBREF]

If you got to this point without a foul I can not see calling one because B1 moves, at the same time if B1's movement cause A1 to move - A1 is not traveling. All of the movments that would be ruled traveling are made by the player with the ball, and not forced by contact with another player, if contact forced the movement it would be a foul.
In this case since it is likely that A1's movement is caused by B1 moving you either rule on that contact which I would have to say is incidental or rule that A1 has not established a pivot point (foot) and ignore the movement.

But the questions here is how did A1 end up on B1's back going for the ball in a scrum without a foul being called in the first place.

it is my opinion that we as officials do not call all lot of the fouls that occur during these scrums, and go with the held ball instead. In reviewing films this year I cam up with 10 - 15 cases where I myself should have had foul calls before the held ball was called as there was clearly a push or hack in the scrum that should have been called first.
That is something I plan to work on over the off season this year.