One thing is clear here and that is that a burn-out does not occur overnight, but over a longer period of time, sometimes even years. Otherwise, there are many possible factors that promote burn-out.
If one follows the opinion of the WHO, burn-out only develops at the workplace, through the external stress there. But there are also other factors that play a big role here. And often it is not only an external stress, but this is still strengthened by internal factors.