Stolen cars is never fun news, and neither is the reliability of Sentry Mode...
There are many reasons that the recording could have stopped:
1 - The car was rebooted during those two minutes
2 - Sentry Mode was turning off while Dash Cam mode was turning on (or the opposite)
3 - There was a system error (e.g. USB reported as too slow - it happens randomly on any type of media)
4 - The occupants were rummaging through the car and possibly unplugged it or caused the USB drive to disconnect and reconnect
and so on...
Unfortunately, there's not one pin point method to say why a recording stopped... In your situation, was the initial recording triggered as a Sentry Cam event? Possibly there was an issue with the second reason above - their approach/entry was captured under Sentry Mode, but, once they started driving, the vehicle disabled Sentry Mode and switched to Dashcam mode, and maybe there was an error, or, when they stopped at the end of the driveway, the vehicle wasn't in Park, and maybe the Dashcam hadn't initialized, or, had an issue starting...