Donkah was out. I don't recall if he started and got injured, or just sat out. Anyway, Rucci was in for him, and it was the guy that Rucci was blocking that sacked/injured Allar near the end of the 1st half. So 3 starters were out in the 2nd half, with the key one being QB.
For whatever reason, PSU seems to do a nice job of making adjustments, on both sides of the ball, at the half. It drives us (fans) crazy that they can't play like that from the opening whistle, but it is what it is. It's why I don't get too frustrated when things don't go well in the 1st half -- I expect the team will play significantly better in the 2nd half.