Presumably some sort of aircraft maintenance issue. Looks like there is another 739 being ferried to TUS from SLC, DL9968, ship 3862.

If you take a look at the tapes for TUS on liveatc.net, you might hear some radio traffic that expands on the nature of the emergency. Otherwise, in time the specific reason may get reported on one of the aviation forums, or perhaps the local news if it's considered big enough.