On of my top bar hives died out over the winter and due to some back problems I just got around to cleaning it out. The hive was fairly clean with some dead bees that I suspect were killed by mites. There was some honey and pollen left but no signs of any brood. I didn't see any signs of wax moths. One corner of one bar, where it appears the final bees congregated, was black including the bees on it. It looked like it had been burned. What could have caused this?