[Ref in PA]

A player, offensive or defensive has a right to a spot on the floor. If the B1 falls to the floor and A1 still chooses to dribble over the top of B1, who has initiated the contact? Who is responsible for the contact? LGP has nothing to do with the foul in this case.

Another example: B1 is stationary guarding A1, with his back to A2. A2 dribbling runs over B1, knocking B1 to the floor. B1 never had LGP, but A2 still charged into him. B1 is entitled to his spot on the floor.

If B1 has fallen to the floor for whatever reason, he is still entitled to his spot. The defender's movement is now somewhat restricted and defensive moves that make contact with the offensive player could be fouls of some sort. You would have to see the play.

Just because a defender has fallen to the floor does not give anyone the right to run him over and have the foul on the player on the floor. The player on the floor still has to commit a foul (ie. do something to cause a foul) to be called for a foul.