Older 54 chip S7s would burn like that all the time, and it was almost always when they lost network connectivity. If you have a very old S7 (first few batches) with very old firmware (which is why you should update your firmware) it could have caught on fire from lack of internet connection. It only takes a few minutes for them to heat up and burn themselves like that.
That may be the case, because when I went to check on them they did not have a connection, but only one of them was burned. Also these were the 45 chip boards. Thanks