I honestly think Sark was hired as an 'insurance policy', in the event Kiffin's problems came to light and a mid-season move was necessary. Sark would have had similar terminology and familiarity with the offense that Kiffin ran, and would've been able to step in reasonably well.
Methinks CNS had 3-4 months with Sark, decided it was more of the same, and decided to cut ties after signing day. Will be interesting to see how this plays out and how quickly a move is made on someone else. I would love to see a bona-fide QB coach hired, but I'm sure CNS is all over this and will make the best possible hire.