I wouldn't say that the Huskers had the game in had when they scored to go up 28-17, but they led in all the good stats, and an 11-point lead 6 minutes into the 3rd quarter is not to be sneezed at.
That onside kick was the critical event of the game. Nebraska basically did nothing after that.
I think that it killed a psychologically fragile team. That team is used to losing, and is especially used to losing close games late. After that failed kick, they "knew" that they were on their way to losing another one, even though they still had an 11-point lead.
Just my opinion, of course.