Most coaches' contracts are up on June 30; public/State Universities have to follow state HR policies and cannot post vacant positions until those jobs are actually vacant (July 1). Even then, some HR personnel behind a desk across campus has to actually post the position, which is a slow process over summer when most University employees take vacation.
So you won't see many postings for jobs until early July.
Then, once posted, there is a period to collect and look over resumes, conduct interviews, makes offer(s), await on acceptance/counter offers, then finally a position is accepted.
This creates an opening at another institution, and the process repeats; sometimes leading up to the start of cross country season.
Thank you Human Resources!