Today's modern seatbelt tensioners have 'controlled explosives' at the base of the mechanism.
In an accident there is a mini explosion in the seatbelt mechanism designed to cinch the belt as tight as possible on you. If there is an explosion...there is a chance of fire.
Courtesy of tireone.com
"Pyrotechnic pretensioners use electrically triggered pyrotechnics that tighten the seatbelt a prescribed amount upon sensing a crash event. These devices can operate on either the buckle or ratchet side of the seatbelt mechanism. These are the most highly technical type of pretensioner, and also the most expensive. These will be the focus of this report. "