Waiting for spring ball is a good point. Players could see where they stand relative to off season development, injuries, better/worse positional log jams. More importantly for some, they could be closer to graduating and getting immediate PT.
Maybe there’s a pattern. December—players who have a chance to join a top team enter portal to get one of last schollies. Jan/Feb—younger guys who see the new class coming in, are not close to graduating, so get an early start elsewhere. April/May—guys where lost close position battles like Burrow, and older players about to graduate.