What can make AA Batteries get hot or explode?

I had four AA batteries (two Duracell, two Energizer) in my pocket for a week, maybe two. They weren't dead, they just couldn't charge my camera flash fast enough.

Earlier today, I felt a warm sensation on my leg and thought I was just imagining things. But tonight just before bed, it felt warm, and kind of like it was stinging. I put my hand in my pocket, and one or more of the batteries was burning hot! I quickly flung off my pants fearing there was leaking battery acid. I didn't see any acid, but one of the batteries was extremely hot.

Why or how could this have happened? Is there are any risk that the batteries could explode? Finally, where should I dispose of them?

I would say they got shorted out by keys or change in your pocket. Or, maybe they moved around and shorted each other out.

This happened to me with a 9-volt battery, which is more easily shorted out than AA batteries (since both polarities are on the same end). There were a few coins in the same pocket. It did get burning hot, and it could have burst. I have never put a battery in the same pocket with anything else since!

I usuallly just throw AA batteries away, though I know that's not ecologically correct. I take camcorder batteries to Batteries Plus in my area - check your Yellow Pages for "Batteries."