Variety of reasons, burnout, management style changing or expectations changing in a way that I can't keep up, I guess those are the usual ones and they're closely related. Most companies also just arbitrarily pick 2 week agile sprints with daily stand-ups, often increasing the amount of meetings as time goes on, and this is usually where things start going downward for me. There also tends to be a decrease in autonomy to get my work done over time, which helps me feel like shit on a daily basis, and so it starts a negative spiral.
In one case I was doing reasonably well with my software dev tasks for a while, but then they assigned me to periodic customer support, and I said up front that's not something I'll be good at. At the same time, my manager tried to increase the oversight of my work, because I was getting slower at it. So I'd wake up every waiting for my manager to "check-in" on me like a child, eventually getting to the point where I couldn't bring myself to get anything done and just resented my existence.
I'm in an ok spot now, for now, but who's to say how it'll be by January. All I want is a place I can do good work for, for a reasonable salary, but it's been very difficult to find.