Originally Posted by Indelaware

Not charter or corporate use. They are in storage, not dedicated to non-scheduled service.

It could be that they are about due for a C check and they would rather return them to the lessor before hitting the actual moment when a C check would be required. Likewise possible that the engines are about to reach the number of hours where they need replacement. Or it could simply be that the interiors are dated and UA doesn't want to invest the makeover. Perhaps UA had been subleasing them to another airline and they need to be re-painted. Perhaps they are scheduled to be returned to the lessor and have been WFU so that UA can remove UA property from the aircraft (perhaps the leasing company owns the frame while UA owns the galley equipment, seats, and IFE system). Perhaps they simply decided that they don't want their capacity and are looking for a sub-leasee. Any number of possibilities.