Here are my thoughts on burnout. Rather than define burnout in terms of the causes (burnout is about stress etc), I think it's better to look at it's effect. For me the effect was simple: I was unable to work. Imagine swimming in molasses - at some point you will simply tire out give up and sink to the bottom. Once you are at the bottom there is no way back out. For me contemplating work was like contemplating trying to swim to the surface in this pool. I became physically exhausted and fell sick.
Burnout is not about absolute workload - there are plenty of times when I have worked an excessive amount and have been completely happy doing so. Burnout is not a binary state, but a landscape of contributing factors forming an overall state of mind where work is not possible. Burnout is a useful handle for a complex story. Some of the things that contribute in my story are:
Fairness is relative. If everyone around you is treated like garbage you will be less inclined to feel bad about being treated like garbage. However if you preceve that people are being treated better then you are you start to ask why - regardless of how well you are treated.
If you have no goals, or if you have goals that you believe are unreachable, you will have no reason to get up in the morning.
Sometimes after doing the same thing for years you just get bored of it and want something more.
Life has a way of throwing random things at you from time to time and this can add stress or reduce energy for whatever reason.
You have been given a job, but not equipped with adequate tools to get the job done. Your hands are tied.
This is not a problem in itself, but if it's there and you are lacking the ability to perform for whatever reason it will make things much worse much faster. Consider pressure to perform combined with perceved unfairness: "why should I work my ass off when I am not treated right?". Or consider pressure to perform combined with lack of goals: "I know I need to do something, but I don't know what to do!!". Pressure to perform can be awesome if everything else is not in a bad state.
Apply pressure to perform, but only once you have sorted out everything else first. These things are:
If it's not possible to do some of these things, for example you have a specific goal and you must have people working on that, then actively and positively try to move them to a different area. It does not need to be a negative thing to move someone on. Even if that means letting them go from the company. If the goals of a company are to extract and burn as much oil as possible and you hire an environmentalist, the fundamental goals of the company and the employee are never going to align. It should not be negative that this is discussed and an arrangement figured out.