BYU traditionally has all assistants on one year contracts, that used to include head coaches, as well, until then end of LaVell Edwards career (it was started so he could assure recruits that he would be staying for their time at BYU). Also, as pointed out, if Sitake has a bad year then assistants will likely be let go, as well. If Grimes did get a "three year" contract, then he likely has a clause that gives BYU an out if Sitake is fired.