The usual culprit is 'replacement' (non-OEM) batteries for an expensive device, for example a drill. The OEM version has a smart charger and a protected battery. A non-OEM battery lacks protection. A non-OEM charger is dumb. Either alone is safe. Combine them, and a fire happens. And because the device is expensive, the people in China who designed the battery and charger never had the original device to do proper testing with.