Legal Guarding Position?

[Huntin' Ref]

Last night I called a pushing foul when a defender "walked into" the post player attempting a shot from behind and committed an obvious foul. This situation had me think about a similar situation that could occur. I looked it up in the case book and rule book and can't seem to string together a clear understanding so I am hoping that some of my esteemed colleagues (you guys) can help me out.

Post player near the block pivots and squares up to the basket. Primary defender is bodied up and vertical. Secondary defender closes down from guarding a player at the wing and puts their body against or almost against the offensive players back. What do you have if the offensive player tries to take a shot but contact occurs as they are essentially "trapped" from behind? Is this a LGP issue?

Thanks in advance!