I think this is how they get a good percentage of their bombers.
Or maybe when they realized that their goal of getting him into the stadium had not been successful, they deemed him useless and detonated his backpack before the Feds could get to him for interrogation purposes.
From what I'm reading the speculation is that the FBI was watching him. If he was unstable and not considered a "real" Muslim (a recruit for their evil deeds) no reason to risk what he knew or how he knew it.