My take on it:

When a cratridge is fired, the brass expands starting near the front of the cartridge and working its way back. Whent the brass expands, it grips onto the tiny imperfections of the chamber which pretty much holds the cartridge into place. The brass will stretch and flow until the base hits the boltface. The action of the brass briefly gripping the inside of the chamber takes alot of pressure off the bolt lugs and bolt face. If the cartridge is lubed, it won't stick to the inside of the chamber as nicely and will slam with increased force into the boltface- giving the appearance of and the same effect and wear and tear on a rifle as an over pressure load.

Also I could imagine a series of lubed or wet cartridges pushing water further and further into the chamber where the moisture or oil gets in front of the bullet. Since liquids aren't compressable, this could in away, act to shorten the freebore of the rifle's chamber. The Free bore allows the bullet to start moving down the barrel and out of the case before the burning powder reaches peak pressure. If you already have a near maximum load and the bullet gets held in the case a few milliseconds longer, the pressure will increase beyond the maximum normal pressure range before the bullet is able to start moving.

That's my best theory on what happens.