Mileage Run Discussion - short-lived, super cheap UA fares

In the last few days, several super cheap (<4 cpEQM) UA itineraries have been "announced" on MR Deals (e.g., WAS-ELP $141 ai; WAS-EGE $142 ai, DEN-SEA $112), only to then disappear within the space of 48 hours or less. What's going on? It seems unlikely that these were mistake fares which UA never intended to publish/sell, though not impossible. Could it be that UA monitors these boards, and seeing the hubbub these fares create, pulls back on them? That too seems improbable, but a number of very short-lived super cheap fares in the space of a couple of days seems very peculiar. Maybe this apparent clustering of great MR fares is a simple fortuity, but why should they disappear as quickly as they have?

Did MRing FTers actually purchase some of these fares before they went poof and vanished?

Maybe the particular fare bucket was bought up.

When there is a screaming deal, only a limited number of seats are available. Believe me, hot deals are not privy to just the FT crowd and they get bought up quickly.