For me it is a lack or presence of interest. I was really enjoying going playing chess again and went all the way to 1571 then I just lost interest and played the games on 'auto mode' while thinking of other stuff and not focusing too much. Some games I would gain an advantage of a whole piece and then just relax and think of other stuff and blunder my way into a loss. I would say online even a 200 fluctuation is pretty normal, depending on how serious you are approaching the games or how much you are enjoying them.
Why is my rating dropping?
I'm with you on that, I sometimes resign after losing a pawn. When I resign depends on the level of the player. However, I never resign if I think I'm against a cheater because of the possibility of them being banned during the game. I also have another suggestion for rating drops: play UNRATED ONLY chess games for a day.

I have MS and received chemo as part of a procedure called HSCT , im 59. I was solid 1000+ with a peak at 1100. Now I'm 829. I have a feeling it is part nocebo, part that I've been rushing and not calmly think as deep as I can. My puzzle rating is in the 2000+. Whether brain atrophy lesions are affecting performance I don't know but I should lay 1 oe 2 games... only a days... I'm guilty of blundering and losing which is forgivable but then stupidly I play 6 more games in a rage usually not much better which such a bad immmature thing to do.. MS fatigue is real so I should refrain from playing when I should rest. Physical exercise is important also! The brain really is plastic. The game is really a game and we should realize playing our best and enjoying the game are two things which go together. Martial arts are similar. Fighters will say that they dont compete well thru rage.

My raiting dropped recently from 1300 to 900. But this is not because of me, i tested my performance via computer in other app and it is the same as before.
I believe this is a change in people who playing in chess.com. when the queen gambit movie came out my rating went up. Because many chess noobies joined with start raiting of 1200... This caused some kind of raiting inflation.... And now when the hype is over... Noobies left... And paying against real chess players, my raiting is dropping....
A few days ago i was a blast And i almost reached 2000 but when I reached 1987 then my rating started going down to below to After a few days 1845 and those days was the worst days and then started to lose the mood of playing and just play some puzzles but my dad forces me to play more chess games and once again i am losing i think this is a sign to take a break this used to happen before so when I start losing, i stop playing and i just do puzzles,lessons u know what I mean , just study and after a few days i started playing and again my rating goes up but now my father doesn't allow me to study and he says to play more and today i am still losing games