If it strays too far from what was initially written. Not the plot, of course, that has to move forward. I'm talking about the heart, what the story makes me feel.
For example, Overgeared made me feel glad to watch as a man who had nothing going right in his life change his attitude and he got better because of it. Treating the world like shit makes it fuck you back, without consent if that matters. Treat it like how you want to be treated and people would respond positively to you.
From the first arc to the latest chapter, the novel stuck to that. It still makes me glad to see the MC happy. His happiness is my happiness, his sadness is mine as well. I grew to love the characters because they feel like a living person.
As for those I dropped, they don't necessarily dip in quality in terms of writing or even how the plot is laid out. I can say that I liked the first few arcs of The Novel's Extra, but it lost my interest after a while.
It didn't change a lot, characters are the same, world setting is the same, but the focus changed over the arcs. From a man who wants to keep things from going bad, as what he did in previous arcs showed, he suddenly turned into a bad guy with the emotions of a child. He regressed into an idiot without any actual explanation or reason, and the heart was lost.
So I drop novels that become longer because the author doesn't know what to do with his characters, making them lifeless and, since they become erratic, pretty much unrelatable.