I think they first became atheists because religion is, well, kinda silly tbh, and dealing with the brain rot of conservative Christianity probably made them feel really confident (euphoric, even) about their own intelligence and critical thinking skills.

Then they were confronted with unfamiliar concepts and arguments from the progressive left, had a negative initial reaction and, thinking of themselves as intelligent and rational skeptics, dismissed them immediately ("these extreme liberals are just as ridiculous as religious conservatives!").

Finally they grew obsessed with "SJWs" and epically owning them with facts and logic and fell into the reactionary pipeline.