Most common cause of fire in gadget chargers
That really depends on the type of charger.

Older style wall warts were nothing more than a transformer and maybe a diode or four if it was a DC unit. Modern "chargers" are sometimes complete switching regulators with complex electronics.

Each and every component can fail. The more complex the more probability of failure. Overheating and lack of cooling can result in sufficient heat to begin combustion. Shorting or arcing can also result in fire.

Overloading can also cook the unit and make it fail earlier than expected.