the buses were an early theory. the latest one seems to be that he tried to enter the stadium with the knapsack, but was turned away by security, he went to the bench to take the device out (perhaps to hide under his shirt or something) - and it went off. check the other "hot" thread on this now.
Blogger mentions 50 spent matches around bench where bomb detonated and suggests it was an intentional detonation. I don't know. People smoke on outside benches. No reason they might not blow themselves up by accident, too.