On April 5 I had to book a flight SFO-OGG-SFO. The outbound on 4/18 was wide open but the return on 4/21 was completely sold out in Y but I got the last seat (full fare). Now that I'm in Maui, I look at the 4/21 flight and this shows up:

F4 Y7 B7 M7 E6 Q6 V6 W5 S4 T2 K6 L0 G0 P4

The loads didn't look like this a couple of days after I booked my flight and the fare changed very little.

Are seats being sold back to UA from brokers?

There may have been some cancellations... more likely, IM may have decided to take a chance at overselling the flight a bit more.

0-7 isn't exactly drastic.

On US frequently flights zero out and then more inventory is added - I think it is to keep fares high and keep selling high fares by only keeping a couple of full-fare Y seats open.

Me thinks conspiracy theories should only be postulated when there is no obvious common sense theory to explain it...this is an ez one. You have hit the upgrade window (100/72/24..hours prior to departure.) People who had requested non-confirmable upgrades have now been cleared based on their status window. So UA cncls their Y reservation and books them in NF, opening up the inventory in Y the previously had.

It makes no sense to zero out seats at Y and B booking classes earlier then they are full. These are the highest Y revenue an airline can get. If the plane is not full, they won't be zeroed out. If your theory were correct, and UA can't sell these seats, the loss in revenue would be way more than they could ever hope to make by doing what you hypothesize....although US did go thru 3 chap 11's in the past year and were bought by someone they never took seriously, so they may use flawed optomization logic