According to Rico Beard of MSU’s Rivals’ site, Mark Dantonio was not happy about the Spartans hiring a search firm to find the next coach instead of himself:
According to @RicoBeard: Sources told him that Dantonio wanted to coach the upcoming 2020 football season, then retire & pick his own replacement. However, in meantime, MSU had hired a search firm to look into finding a new coach. Dantonio found out, was upset, & abruptly retired
— Mike Sullivan (@MikeSullivan) February 11, 2020
MSU has been turned down by several coaches already.