when they see who the new coach is and have a chance to be pitched their role on the team by that new coach.
Several will leave for good, and the new coach will go recruit “his” players to replace them, which is one of the nice things about the transfer portal.
But if I was a BYU basketball player right now I would absolutely be entering the portal to at least see what else is out there and develop options in case the new coach is a dud and/or he doesn’t really want my skill set there.