Speaking from experience here...... they CAN explode. No, it does not take something to hit the eggs. I have had a rotten egg explode in my incubator. Believe me, you don't want that to happen.I cleaned out part of my old side of my chicken coop that has been sealed off for over a year and found a few eggs buried so I poked one with a shovel and it made a "pop" noise like a small fire work (smell was really bad), so I'm thinking maybe something has too hit the eggs to cause them to explode, because if it had pressure in it for that amount of time and didn't go off the incubator eggs might stand a good chance of not exploding.