What you see as a fare change is often not a real fare change. It is filling up a fare class so the fare you see a bit later is the fare for the next higher fare class. Say the site has switched from showing you U fares to showing you M fares. Neither the U fare nor the M fare has changed by a penny, but you saw the U fare before and you see the M fare now because there are no U seats left. It looks as though the fare has gone up.

If the flight sells more slowly than expected from here on out, the airline may make more U seats available to encourage sales. It will look as if the fare has dropped, but it hasn't.

As far out as you're talking, no fare classes have sold out yet though some may not yet be available. So, nothing seems to be changing.

If you ask your Toyota dealer "what's the lowest-priced car you've got?" and they have a Yaris on the lot, they'll give you a low price. If they sell it and you ask the same question tomorrow, they'll have to give you the price of a (more expensive) Corolla. If they get another Yaris in next week, they'll go back to quoting that one. Same thing. It's not as if the price of a Yaris or a Corolla changed.