When is the flight? If it is within the next 3 days, then you may have just witnessed the opening of an e-500 window, when any GS/1K (or 1P/2P if the flight is within 2 days) members would clear their pending e-500 upgrades, as the e-500 window for GS/1K/1P/2P is 120/100/72/48 hours prior to departure.

If the flight is farther away than that, then it could be UA shutting off upgrade inventory in the hopes of getting some revenue tickets... I've seen that happen, as well. They would then probably open the upgrade inventory back up a few days before the flight (or at the gate).