When re-heated in a microwave, why do Brussels Sprouts explode?

When microwaving, water molecules expand evenly in the item that is being microwaved. If an item doesn't have a skin or a shell as a barrier, the molecules on the inside of the object expand so rapidly that the outer edges cannot contain them and thus an explosion occurs.

In the case of leftover Brussels sprouts, they were likely saturated with some kind of liquid in the original cooking process. Therefore, there are more water molecules in the leftover sprouts themselves and so they are more likely to explode in the microwave.

http://www.repairclinic.com/RepairHelp/FAQ/55/Microwave-Why-Do-Eggs-Potatoes-And-Other-Foods-Explode-When-Heated-In-A-Microwave