This is what burnout is. Burnout is not caused by working, but by stress. The root cause of this stress is personal and interpersonal factors. People can burn out on very small workloads if the ratio of interpersonal toxicity to hours of work is high enough.

Sometimes it looks like people are burning out because of extreme workloads. In these situations, I've always asked "why is the workload so extreme to begin with?" This gets you to the real source of stress.