There are eyewitness accounts that he tried to enter the stadium but left when they tried to search his pack. Then there's the account of the bus driver, who saw him go to the bench, sit down, fiddle with the pack, and then explode. My guess is that he realized the only way he could smuggle the bomb into the stadium was to hide it under his clothing. So he picked the bench as a quiet spot where he could transfer the bomb from the pack to his clothing without anyone seeing him. And while he made the transfer, he accidentally set the thing off. Woops.
This latest update mentions that he was wearing a vest with wires sticking out of it.
http://www.ktul.com/news/stories/1005/266002.html
do you have any kind of source for that? I am having a discussion on a OU football board and would like to use that if I can have a source for it.
Very similar to the London bomber whose pack (likely mistakenly) exploded on the bus.