It could be the capacity of the headset. The battery of the headset is only 100mah small capacity, and even smaller capacity is available, and the general standard charging current is 100mA.
If you use an ordinary mobile phone charger to charge directly, even a 1000mA charger, it is 10 times the standard. So the battery will be a lot of heat, it is easy to burn.
It was caused by internal component damage, short circuit.