I agree this is the most likely cause. If that is the case I suspect that what is inside that case is quite unstable. So don't even think about using a kinetic bullet puller or you may find yourself picking shrapnel out of your body parts. I also would not store it in any area of my property that contained something I wanted to have intact. While just one spontaneously going off is unlikely to set a gas can in the area on fire I wouldn't be so sure about 10 or 20 going off.