Some examples:
"An atheist killed someone. They killed them because they were an atheist."
"A man strangled his wife. He is clearly a misogynist."
As you can see, one variable has been extrapolated and used as the causality, whereas in reality there are hundreds of cross-pollinating motivating variables.
This is primarily a genetic fallacy: you are deciding that origin or position explains outcomes. But then you are layering on the assumption that correlation implies causation.
C.S. Lewis suggested this use of a genetic fallacy to hide another simple fallacy was so common in use that it needed a name of its own. He decided to call it Bulverism.