It's because they were supposed to get home games from the Jamaica Classic, but they weren't able to hold up their promise on those. By the time that USU realized they were never going to get those games, they couldn't get anyone to come to Logan in D1. The rumor from the school is that they literally sent offers to every D1 in the nation, but no schools could come by that point. I don't know if I buy that completely, but I don't doubt they couldn't find a D1 to come.
They even sent out tickets prior to getting these games finalized. Season tickets say game on this day, opponent, TBD. St Katherine's got added to the schedule on 10/22 and Eastern Oregon on 10/30.