Perhaps the bomb had a timer or a remote detonator.
All I know it had a directional device and have learned since they found metal in the surrounding area and trees. Just wanted to blow himself up -- yeah right. He either got blown up by accident or people with him blew him up as he was out of control. He failed his mission thanks IMHO to the alcohol policy at OU where they REALLY check bags of everyone coming back into the stadium near halftime.