A more likely senerio is that he wasn't able to get the backpack in, so he was removing the explosives and hiding them on his person for another try when one of the wires touched and caused a work accident.
"...It's obvious Hinrichs depression was a motivating factor as he ultimately didn't detonate in a crowded area."
A map of Norman indicates he had to go 9 or ten blocks from his apartment before he got to the spot where he ultimately blew up - right next to the stadium. And there are conflicting reports as to whether he actually tried to get inside the stadium.