Why do eggs in the shell sometimes explode when cooked?

I pressure-steamed a dozen eggs this morning. When I was running the ice-bath, I found this one. Anyone know why this happens?

I guess there must have been air inside the shell that expanded when heated, but why?

I have never pressure steamed eggs before. I have only boiled them.

I have had this happen in the past. I also wondered why. It sounded like it is was usually due to too fast of a temperature change, too hot or fast or too long of a boil. Or possibly one of the eggs already had a small crack that was not noticed.

I don't believe I've had it happen since I've adopted the boil for a minute, cover, turn off and wait 12 minutes method.

I have only boiled them as well, but I suspect a weak point or micro crack of some kind in the shell probably allowed that to happen.

Temp shifts in either direction too quickly can cause cracks. Bumping into each other while boiling can cause cracks... but in all of those cases I suspect that the shell was weaker than "normal" which is why you see that happen.