Without knowing ehat route and when it’s hard to drive xpkain.

a problrm is if the route you are looking st say from Chicago to lax you coukd have a big business meeting planned and set so corporate HQ booked flights where peop,e from 30 cities in east are all rooting to Chicago and bring on the same flight or it’s a planned corporate retreat out of chicsgo where you have 40 corporate heads travel.

as others said it could also be a summer field trip bu students or a large family is traveling for a wedding.
there also could have been an equipment change where 5 weeks out instead of flying a 200 seat aircraft they changed it to a 125 person aircraft because it was only at 20% fill on original plan, after plane switch its 60% full.

ftom my experience...

mist domestic flights its 4-5 weeks out when everyone books travel plans. It’s whem fares tend to be lowest in average. There are exceptions to this and they are..

1 larger travel destinations for planned vacations like Orlando where peop,e book linger out
2. For some routes like coast to coast can go up 2-3 months out because so many people book the second leg through Chicago hub to thus destination.
3 limited service options. Prices between say nyc and la will likely go higher faster on an airline like Alaska which doesn’t have as many flights and routes to the west Ciara from nyc than an airline like United where besides direct it has connections through many cities.
4 you see this in reverse on southwest where they have non hub direct flights that can be much cheaper than through a large connection point like Chicago.