Three possibilities IMO:

1) Flight is very full and sold out up front, which I'd say is unlikely. How does the seatmap look?
2) UA is toying with canceling the flight during an upcoming schedule change and wants to restrict bookings so that it doesn't have as many displaced pax to reaccommodate.
3) A random glitch. It has happened to me before, and within a few days inventory goes back to normal.