Legal guarding position?

Player A1 has the ball and is dribbling towards his basket on a fast break. Player B1 is in front of A1 and is running down to meet A1 at the rim. Player B1 never establishes LGP. As they approach the rim, B1 is defending with his arms vertical, and his back is to A1. A1 initiates contact with B1 by lowering his shoulder to create space for a layup, and barrels through B1. My partner calls a PC foul on A1.

I thought it was a block because the defender never established LGP, regardless of if the offensive player initiated contact. What is the correct ruling for this situation?