It happened to me a few weeks ago. I went through three tubes (which I saved for patching later), then realized I had a small hole in my tire. The threads were poking through a little bit... so being that they were old, cheap tires anyway, I bought a new one and didn't have any more explosions. I guess the tube was being pulled, pinched, or punctured by those little threads, or maybe even tiny shards of glass. The hole in the tube lined up perfectly with the hole in the tire too. I can't think of any other logical explanation.

But yeah, could be a pinch flat too