I assume you are referring to the sensors mounted at the bottom of the track. Part of the vehicle could be under the door before the front or back wheels interrupt the light beam of the sensors sending the door back up. Unless you have some non-standard set up , the door would only start down if the opener remote or wall button was pushed.

Who initiated the door closing or was it on a timer? Did the door strike the car between the bumper and the wheels or after the wheels had passed through? Did the door retract after striking the car? Is this a residential type GDO or one for a commercial garage?

Clearly, something didn't work right