I read a long time ago, that seating a bullet too deep can double pressures in some cartridges, especially 9mm.
This was from a Speer reloading manual that I haven't seen in years. It was in the section on straight-walled rimless cartridges, because that type of cartridge head-spaces on the mouth and only a tapered crimp can be used. The concern was that if the crimp wasn't tight enough the forces of slide action during firing could force loosely held bullets of rounds remaining in the mag, to seat deeper, causing a potentially serious over-pressure situation.