foul from behind
 

A1 and B1 are out ahead of the ball on a fast break, followed closely by A2 with the ball. B1 stays with A1, so A2 takes it all the way to the hole. In the process, A2 runs right up the backside of B1 with lots of contact. B1 is busy guarding A1 and doesn't even see A2 coming.

What is the call?

I called offensive foul since A2 was completely responsible for the contact, but there was never legal guarding position. Did I screw up?

Nope, you did fine.

LGP is not required for all situations.

Remember that each player is entitled to any spot on the floor provided that he gets there first and without illegally contacting an opponent to do so.

Your presented situation appears to most closely resemble a screening situation in which both players are moving in the same path and direction. In that case, the player in the rear is responsible for the contact if the player in front slows up or stops.

Since, in your situation, the player in the back had the ball, this became a player control foul.