Fares are very dynamic. And carriers have multiple ways they can change them.

Someone has touched on ATPCO, and the fare filing times. These fares can be changed anytime, and uploaded during the next filing. These can range - they can totally eliminate the fare, they can change the price, either up and down, of the fare using that same fare code, they can add a surcharge for certain days of travel (for example, on peak days, they may add $X to the fare just before/after holidays, Fridays in summer, etc....listed in the fare rules, sometimes they even seem mostly random to me, etc.). They can change the info. in the rules so the advance purchase deadline might change, from 21 days to 30, for example. Or they may just drop or add fares.

Inventory management can also just 0 out buckets. So I've seen flights I'm looking at be, say, T9, and then a few hours later go to book, and see its booking into W not because they dropped the T fare, but because they closed out the T availability and W is the next fare that meets my travel conditions. They may re-add it again later, or maybe not.

Best thing to do - if you see a fare you like, book it - immediately. UA has their 24 hour flexible rebooking policy - so you can always do your due diligence, then if you decide to cancel, you get everything refunded as long as its within 24 hours of purchase. I also like using Farelock when it makes sense - its available most of the time on UA/UAX operated flights, and for a flat fee, you hold that fare for either 3 or 7 days. Sure, I've lost on that at times (fare drops or is the same by the time I want to book), but overall, it's saved me probably hunderds of $ over the years.