Why do lumps of meat, esp chicken, tend to 'blow up' when reheating meals in microwave?

It's steam from water inside. Mine does this. I now lower the power and stir a couple of times. Seems to stop it.

Never reheat on full power and only use a microwave if you really need to, it's far healthier and safer to reheat on a hob on a low to medium flame/setting.

It’s a BLEVE

Boiling Liquid, Expanding Vapor, Explosion. An at home physics lab.

Water/moisture pockets inside the meat

I use toaster ovens at home

All that a microwave does... the only thing it can ever do... is boil water.

By boiling water inside the chicken you're creating pressure that explodes.

Maybe try less power. Cooking slower.

Steam? The moisture in the meat boils, expands, POP!

microwaves don’t distribute this energy uniformly. Certain areas of the food, especially thinner edges and corners, tend to heat up much faster than thicker, denser portions. This uneven heating creates a breeding ground for bacteria.