[londons_explore]

One contributor is that most batteries have a protective circuit built in. It usually has a DW01 chip and a pair of MOSFETs to prevent overcurrent charging, overvoltage, undervoltage, and overcurrent discharging, all for under a cent.

Since most batteries are protected, the cheapest chargers can be 'dumb', with a simple constant current output. When the battery is charged, the built in protection device will stop the charge when charged.

However, only most batteries have this protection. If you plug one of these cheap dumb chargers into a battery that doesn't have the protection circuit, then it will catch fire after a few hours.

[londons_explore]

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.