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The highest pressure is typically right at the point before the bullet starts to move. Handgun powders burn pretty fast, so that pressure peak is reached rapidly. Rifles uses a slower burning powder, and tend to have a longer time of high pressure. But, since a handgun has a small barrel, you need all that pressure fast to get the bullet up to speed before it leaves.

The .40 is a "high pressure" handgun round, that is a lot higher than say a .45 ACP, so there is less margin of error. Compressing the bullet in a loading with fast powder makes the pressure go up a lot, sometimes enough that the case will rupture before the bullet has moved far enough to allow the pressure to go down. There just isn't enough room for the burning gas, so you get a Kaboom.

Glocks, it seems, are known for this, rightly or wrongly, but it can happen in any handgun or rifle. I saw a similar (but less dramatic) thing happen with a 1911 once.