Regarding tortious interference, it would be a spite suit, depending on assistant contracts.
If they're like most schools, they're on one-year contracts, so they probably couldn't prove much in the way of damages because there's little expectation of entitlement to their services beyond the existing contract. Also, without their hiring coach at the top, their services aren't terribly valuable. But they could fuzz up an argument maybe that a new coach would have wanted the choice to renew their contracts.
Then again, every time a new coach is hired away, their contract is "interfered with" by the hiring school and same with any assistants he brings with him.
The breach of fiduciary/employee duty would probably just generate cause for firing. But that would require him to be fired before he resigns, most likely.
Probably all this would do is make a mess and make UM a less attractive employer. And what they can't ultimately do is force him to stay at Ole Miss to complete his contract.
This kind of thing could come up in every hiring scenario, although it is somewhat unusual for the coach to "announce" or "threaten" like Kiffin and I suppose some damages could result from that, taking it out of the usual scenario where no one uses.