What caused my watermelon to explode?!

What caused my watermelon to explode?

I bought a baby watermelon the other day and left it on the kitchen counter. I came home today and there was a LOT of awful smelling liquid around the watermelon. It was all foamy, so I thought maybe it was dish washing water from the sink. I inspected the watermelon and notices a small (about 1cm) crack at the spot where the stem used to be. Liquid was slowly FIZZING out of it. I cut it open to see what was going on inside, and it almost exploded. It fizzed like crazy and sounded like you just opened a soda bottle that was shaken up. It smelled awful! It freaked me out, and I don't know if I can ever eat watermelon again! Can anyone help me understand what was wrong with that watermelon?

It was rotting or fermenting inside... the bacteria finally produced so much gas that the pressure inside caused the shell to rupture. The air escaping just pushed out some of the rotten liquid with it.

I was going to make fruit salad about 2 weeks ago and brought a baby watermelon. When I cut open the watermelon, it smelled really gross... almost had a rotten meat smell. I guess if they sit too long in the store, they have a tendency to rot & ferment from the inside.