I agree, I think it's always some form of BPA. The way I see it is when they're down in the later rounds and over 600 guys are off the board no one has a definitive board where player x is the BPA, and player y is BPA available after him. They have guys in tiers of talent. I think at that point it's easy for a team to say we need HS pitchers because we have no pitching depth in Salem/GCL etc.
Of course, going from 40 rounds to 20 rounds may change things.