I've worked some tough jobs in my life. Physically laborious. I'm talking 100+ hour weeks on an ice cold production line working under complete psychopaths. I felt overworked, abused, frustrated - I turned off my brain, chatted with coworkers, and did the job. But never once did I feel "burned out". No matter how much I hated the jobs, I could always stand there and do another day. And the end of every shift felt so good.
In contrast, I have felt very burned out in relatively nice jobs - thoroughly pleasant environments with minor workloads. I don't even know why. Something about staring at screens all day - something about that "unplugging" feeling you get after locking in on some code all day - maybe constantly thinking about how other people think.
Honestly, the closest non-work simile I have found was signing up to do a video game marathon. After hour 8 I had a distinct feeling of burnout for that game - as bad as any terrible job I have ever had. For doing something I should otherwise find enjoyable.
So I think there can be some amount of trauma involved, but I think the core of "burnout" has less to do with actual negative experiences and more about the type of work we engage with. And I specifically think it has to do with our capacity to either do continuous creative problem solving, or engage with large varieties of different people - both activities that humans have not historically had to engage in for extended periods of time in previous eras.