Pure guesswork, but one possible hint is that, if all the academic seniors graduate, it would leave the Paladins with only 4 guards on the roster, and that's including Foster, who is really more of a 3. The Paladins could sign a guard in the late period, but (1) that has not been CBR's M.O. and (2) it's not ideal to rely on true freshmen for depth anyway.
There's no need to take graduate classes unless someone chooses to graduate first, which wouldn't have happened yet. So 4th-year seniors can just choose not to graduate next Spring if they want to come back for the 5th year.
For Bothwell and Slawson especially - they both could have solid playing careers overseas, and might benefit from the extra year of development before hitting the market.
Of course they could grad transfer also.