The Verge reports:
The vehicle logs confirm that the automatic Summon feature was initiated by a double-press of the gear selector stalk button, shifting from Drive to Park and requesting Summon activation. The driver was alerted of the Summon activation with an audible chime and a pop-up message on the center touchscreen display. At this time, the driver had the opportunity to cancel the action by pressing CANCEL on the center touchscreen display; however, the CANCEL button was not clicked by the driver.
The Summon feature enables the car to start up and drive itself to you from its parking spot. If the data is to be trusted, the Model S made it clear Summon was engaged, and by getting out of the car without canceling Summon the owner neglected to keep the vehicle from autonomously crashing into the trailer, hard enough to smash the windshield.
The owner, Jared Overton says he stood next to the vehicle for up to a minute before leaving, and according to Tesla the vehicle started moving three seconds after Overton exited the vehicle – with the logs to back it up. The way the stories differ means that from Tesla's view, the owner must have seen the car take off immediately after closing the door (and failing to cancel Summon) – not going rogue while he was away.