All Shuttle A320's are now serving more lucrative high density routes in the mainline system, as they are all one-class and can hold more passengers thatn standard 320's. Some Shuttle A319's are doing the same thing, and they needed an aircraft to fill the void, so they chose the 733, which works well for the short frequent hops that Shuttle does.
Just for the record, all of the Shuttle 319/320 aircraft that are now doing mainline flights still have the one-class configuration, and still have the Shuttle livery, and the 733's doing Shuttle are still standard two-class, with the basic US Airways livery.
By the October 26 schedule the all coach 320's will have been converted to a normal two-class configuration (F16-Y126). Also one 320 is sometimes used in the LGA-DCA and LGA-BOS markets replacing a 319.