I'm guessing we've also got more financial obligations than any other time in school history (that construction site directly south of the stadium will serve as a reminder all season long). A 5-7% dip in season tickets sold, even if it's from a landmark high, is nothing to sneeze at.
EIU fans can and do buy ISU season tickets to get into the cyhawk game. A decent amount of drops are likely because its a year without them on the schedule.
But ticket prices are about to shift upward they do not want to charge people for the new end zones what they currently pay on the hills or the bleacher. And only the very ends of the current stadium are low priced.
The drops won't necessarily be the problem if the season is bad the problem will be that they will have to under price a lot of the stadium to keep it filled when they want to raise prices to reflect major construction.