One random example, picked a route with low avg LF.
Route B77525, WAOO - WARJ.
Game date: 1998-03-15. Your current Y class price is $218 while system recommends price to be $145. LF is hence only about 25%.
Backup, game date: 1998-02-18 (14.1. / ~14.00). Ticket price for Y class is still that same, $218.
Backup, game date: 1997-11-14 (12.1. / ~14.00). Ticket price for Y class is still that same, $218.
So they definitely do not change by themselves .. I would suggest to check what you do with the pricing tools and how you update the prices, since I think you may be adjusting them without realizing.
This particular route had a near-full load factor still a few game weeks ago, with that $218 price. It was just fine since you had a monopoly there and supplied less than the demand. But the competitor has just opened on this route (at the end of Feb-98) and it means that there is now more supply than demand, so your high price doesn't cut it anymore. = The low LF is not because of prices changing by themselves but because there is new competition around and your old lucrative routes cannot be flown with "monopoly pricing" anymore (pricing which you've set at some point in the way past; at least on this particular route).
(Also the reason for your reduced profits are NOT reduced revenues (ie. decreased sales). Your expenses have steadily increased all the time over the last few years - see income statement and compare the graphs there.. For example on 7/1996 your operating revenue was $351mil and ops expenses were $325mil, and in 12/97 revenue was $346mil and expenses $354mil.. So besides of finding ways to increase the revenue, I'd suggest taking a good look at the expenses too.)