I think it is the natural wear of the season coupled with the fact the D is on the field much more that the offense. That is due to the ineptness of the offense at times and the bend-not-break philosphy of the D. Since the beginning of conference play Nebraska has only once ran more offensive plays against the opponent with that being the Texas Tech game. The stat from those games: Missouri 78 NU 66, TTEch 57 NU 68, ISU 67, NU 65, Baylor 70 NU 59, OU 87 NU 57, Kansas 70 NU 63 and most recently KSU 67 NU 64. Even being great athletes the big boys on the D Line are going to wear down going through that.
One example of how a body cannot withstand such as beating is in the NFL and backs that have carried more than 400 carries in a season. Nearly every single back that accomplished that feat never had productive seasons after that. The most recent being Larry Johnson.