I'm wondering if it's related to all the weird double flights in May (UA and CO flight with different flight #'s, aircraft types but same times) and/or the SHARES integration.

I had an itinerary that I checked yesterday that has travel on 5/8 from IAH-SFO. My flight at 7:44am was originally a UA flt # and there was a CO flt leaving at the same time. Yesterday it was offering premium seating for $300 on that leg (didn't take it), but this morning the UA flight is gone and I've got a schedule change to the CO flt. Logged in to the pnr after the call centre had updated the reservation and premium seating offer is gone.

Could be the load factor (although I doubt it), but I have a feeling we'll be seeing a lot of this in the next couple of months.