Help me work through a block/charge scenario.

[CoachCER]

This happened last night in 7th-8th grade boys rec game. One of the teams in question have an issue with the referee who made the call based on this incident and past ones. Their "formal complaint" is being dismissed by me, but I am trying to educate the coaches involved as to why the call was made. The referee in question is very solid, calls a lot of varsity games, but sometimes leaves himself open to these situations as he is a little too willing to try and give an explanation during the game.

Here is what I saw:

A1 catches a pass just inside the top of the key, pivots, drives hard to the basket. He does a jump stop, and makes an explosive move to the basket with his shoulder lowered. Think of a move where he is exploding out, and then up, in one motion from the jump stop.

B1 slides in late under A1, hard contact occurs, and both players go down hard. B1 was absolutely punished by A1's shoulder.

Based on what I have described, and assuming B1 did arrive late, what do you have and why?