Sadly, grades are only one possible reason for any player, others include health, personal reasons (such as family illness, etc.).
If you are really that curious - just look at a Penn State game to see if she is on the bench, dressed or not, etc. And listen to the commentators who sometimes say things somewhat out-of-turn, although not always correctly. A player (transfer) for RU was announced out for the season however she recently returned. The unspecified injury turned out to be concussion protocol.
Other "we don't really need to know examples":
On the men's side here at Arizona regarding a player not playing. He eventually explained that he had been given a banned drug unknowingly, and the NCAA agreed with the circumstances and imposed no penalty, except, he couldn't play until there was no trace of the drug in his system. He's playing now. Plus Tyler Scaife, for example, who hasn't revealed her "medical procedure" that is keeping her out this season, but felt it necessary to explain to all and sundry that it was neither having a baby nor having an abortion. Because the speculation would have been . . .