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.
I think it's more likely, given the nature of TATP, that he detonated the bomb by accident, while trying to arm or disarm the bomb, or while trying to transfer it from his backpack to inside of his clothes.
I don't rule out remote detonation by watchers, I just think it's less likely.