I've seen that as well. It happened to my Dad, he doesn't bike a lot, but goes for bike rides with me when I am in town. He gets nervous on descent's so tends to ride the brakes. Well last time I was visiting he was riding the brakes on a long down hill, finally BOOM! Flat tire. It was an old hook-less rim, and was probably on the edge of being overinflated as it was so a little increased temp. & increase pressure did it.