I fly WN out of TUS more or less every week. I've seen them do this from time to time, usually if the flights are empty. TUS is not like LAS or PHX or other stations where they give you 30 minutes of warning and the herd starts lining up before the plane is on the ground. Usually they make the announcement once people start getting off.

If everyone's there and accounted for, why sit around? That's the beauty of TUS--they'll often push early if they can.