And this isn't the first time it's happened this year.
I was curious about this, so I went back and watched the other kick attempts in the Northwestern game. There were a total of 5; 3 by Nebraska and 2 by Northwestern.
One thing I noticed, at least for that game, not sure about other games since I haven't gone back and looked, but it appears that Nebraska lines up for field goal blocks vary wary of this penalty, which may be one reason why Scott Frost has commented on this. Nebraska stacks both sides of the long snapper (LS), but doesn't have any down linemen directly over the LS. There is a standing player over the LS (in the case of this game, Will Honas on both attempts) but he appears to be there more for sniffing out fakes, as he doesn't make much attempt to block on either kick (again, probably wary of this penalty). On both kicks, there is contact with the LS by a Nebraska lineman; but it's both initiated by the LS and comes after the 1-second time frame defined.
For Nebraska's 3 field goal attempts, Northwestern lines up differently, having a down lineman directly over the LS on 1st attempt, and one directly on either side of the LS on the second and third attempts. On the first (made) attempt, that lineman actually angles toward his left, to the right of the LS, and initiates contact with the Nebraska lineman next to the LS. Nothing illegal that I can see on that play. On the second (missed) attempt, instead of one guy directly over the LS, they have two guys lined on either side, basically on each shoulder of the LS. Again, with the ball snap, nothing illegal appears to happen. The player over the left shoulder of the LS takes a step back and kinda stands up, probably watching for a possible fake. The player over the right shoulder of the LS does drive forward, but first makes contact with the player to the right of the LS, before then being contacted by the LS, which is legal.
The last one is already discussed above. Northwestern again has two players on either shoulder, but this time they, as well as multiple other players all make a drive directly for the middle. It's hard to see from the behind the goal posts angle which player makes contact first, and I can't find a behind the kicker angle right this second, but multiple Northwestern players drive directly at the LS and he quickly ends up laying on his back.