How should this situation be handled?

[walter]

Could be a push, could be intentional, could be flagrant. I'd have to see the play. If it was simply jostling for rebond position it would just be a push. If B pushed A5 into A1 or pushed him/her forcibly out of the way, could be intentional. Another possibility is if A5 was pushed into A1, could be flagrant on B. The penalty depends on how you see and judge the play.