It looks like things were just too bad in Lincoln after Nebraska's upset loss to Georgia Southern that they could not wait three weeks for his buyout to substantially drop from $15 million today to what would have been $7 million if they had let him stay till October 1 :
www.cbssports.com

Nebraska fires Scott Frost: Cornhuskers pay massive buyout to dismiss coach three games into fifth season
Frost was unable to find success in four-plus seasons at his alma mater
