Del Rio received a very good deal to return. There are conflicting reports on if Triple H contacted Del Rio or if Del Rio approached WWE but apparently Del Rio has been the planned opponent for Cena's US Open Challenge all along.
The deal to bring him back to WWE was reportedly done weeks ago but Del Rio was giving the impression that it wasn't done weeks ago. However, people in WWE have known he was coming in. Del Rio, the current AAA Mega Champion, will reportedly work other dates through the end of November but that also hasn't been confirmed. AAA officials reportedly didn't know about his WWE return until Sunday some time.
Del Rio's new WWE schedule is said to be easier than most talents but not as easy as Chris Jericho, Brock Lesnar or The Undertaker's schedules.