The details I've read are sketchy. But I think Alexander's eligibility issues center around his family receiving improper benefits from a third party.
Buford was an entirely different story. He and OU's coaches thought he had taken and passed all of his requirements to play this season. They were notified after he was already on campus that the NCAA was challenging one or more courses on his transcript from Arlington County Day. OU, Dante and ACD lost the appeal and Dante was ruled ineligible to play this year. Judging by everything I have read and heard, none of it was his fault.