The thing is, people who create drama tend to be much more visible than those who suffer quietly (for better or for worse).

Presumption of innocence implies that the default attitude towards accusers is healthy skepticism plus a willingness to listen. This is necessary because if we are not skeptical nearly anyone can successfully attack anyone else with impunity and destroy their reputation and if we aren't willing to listen we risk ignoring victimization.

In this case the proposed victim is silent on the matter and there is not only no proof there aren't even any details. My default assumption of skepticism leads me to conclude in the total absence of any other information that the individual was fired for some other reason and the accusations of bias was erroneous.