Wisconsin last year was a disaster. But controlling for that game, which had a RB of much higher quality than any faced this year, the stats in run defense end up being quite similar.
Neither proving nor disproving this allegation about Wisconsin, I looked at the numbers. If this exercise proved anything, it's that putting numbers into a spreadsheet is always entertaining.
Including Wisconsin, 2014 Nebraska faced 487 rushes, giving up 2,357 yards. That's 4.84 yards per carry.
Excluding Wisconsin, 2014 Nebraska faced 434 rushes, giving up 1,776 yards. That's 4.09 yards per carry.
2015 Nebraska faced 380 rushes, giving up 1,428 yards. That's 3.76 yards per carry.
Excluding 2015 Nebraska's worst rush defense game (Purdue), Nebraska faced 343 rushes and gave up 1,245 yards. That's 3.63 yards per carry.
An apples-to-apples comparison, total games vs. total games, shows the 2014 Huskers gave up 1.08 more yards per carry than the 2015 Huskers.
A second apples-to-apples comparison, worst game taken out for both, shows the 2014 Huskers gave up 0.46 more yards per carry than the 2015 Huskers.
2014 Nebraska held two opponents under 100 yards rushing.
2015 Nebraska held five opponents under 100 yards rushing.
2014 Nebraska held eight opponents under 150 yards rushing.
2015 Nebraska held ten opponents under 150 yards rushing.
2014 Nebraska allowed four opponents to rush for 5.0 or more yards per carry. Nebraska was 1-3 in those games.
2015 Nebraska allowed three opponents to rush for 5.0 or more yards per carry. Nebraska was 0-3 in those games.
2015 Nebraska faced 107 fewer rush attempts than 2014 Nebraska.
The 2014 Wisconsin was weird. Granted, they've always had Bo's number when it came to rushing, but even considering that, it's an aberration. Wisconsin rushed for more yards in that game than Nebraska's previous four opponents, combined. Sparty ran for 188, Northwestern for 117, Rutgers for 143 & Purdue for 124. That's 572 yards on 147 carries, an average of 3.86 YPC. Wisconsin ran for nearly 11 yards per carry.
These two defenses were very different animals. The recipe to beat 2014 Nebraska was to attack the edge in the run game. The recipe to beat 2015 Nebraska was to throw the ball. Very different defenses, very different plans of attack faced by each.
EDIT - there's some inaccuracy in these numbers. I must have keyed them in incorrectly, so my apologies. Mav has what appears to be more accurate numbers in this post below:
http://www.huskerboard.com/index.php?/topic/78018-collins-declares-for-nfl/page-2&do=findComment&comment=1659303