I dont think the harness came loose man, it was locked, just not in the most closed position possible. Look at the photo the shoulder restraint was never in the fully closed position due to his size and stomach getting in the way, looks like it could have went another 12" to be fully closed. The shoulder restraint is suppose to come all the way down and lock against the seat near your pelvic area. Here is a photo of him before he went up, his seat is NOWHERE near being fully closed, compared to the rider on the right side of the photo. He went up, the ride tilts you forward before dropping and he slipped right the fuck out due to his weight and laws of gravity.
IMO the ride operator should have never allowed him to get on the ride based on his size and weight, along with the seat not being fully closed to the pelvic area.