Sometimes I just get bored with a fic. Usually if it takes too long to update, and by the time the next chapter rolls around I've lost interest in the fandom and/or forgotten everything that happened in the fic. I generally don't like slow pacing either, so if I'm itching for something interesting to happen and the story kinda stalls for a while, I'll probably drop it. (Though if it's completed, I can skip ahead through the "filler" if I'm generally into the story otherwise)
Or it had an interesting premise/tags/summary but doesn't really deliver. Either the characters veer into OOC, or worldbuilding/magic/fantasy things integral to the story aren't explained so the reader understands what's going on and why. Though I guess those are more "bad writing/storytelling reasons."
Outside of that, untagged tropes/squicks. For some of them, a brief mention is fine, but generally if the story starts heading in that direction or focuses mostly on those things, I'm out.