They may have thought he was going to ditch the backpack and squeal on them
Exactly. With his head down on the bag, he may have been using both hands to try to disengage some type of dead-man switch. Either his watchers could have detonated it, to prevent him from squeeling, or he detonated it himself accidentally while trying to disengage the switch.
If I had been one of his handlers, I simply wouldn't have let him go all the way to that bench if I thought he was getting cold feet. It was far enough away for me to suspect he might be getting cold feet before he got even halfway there, and after seeing him try to get into the stadium three times and fail, I would have been ready to detonate him at any moment. But if I had waited for him to get to the bench, to see what he was up to, and saw him taking out the bomb, I would have realized he was going to try to sneak it in under his clothes, and I would have let him try it and not blown him up.