I suspect a whole lot of atheists were brought up religious. The heavy religiosity is the push they need to even think on the subject. I think a lot of people who are what I'd call passively religious (non-practicing, don't really care, but might say say they believe in god if asked) don't have to engage with the material critically, so it's not as much a part of their world. For sure there are atheists out there who have a dogmatic approach to atheism because of their former belief systems

But even beyond that, I think it runs deeper. Christianity, if you're in the west, is foundational to our culture, even in secular nations. It still informs traditions and morals and perspectives that can trace themselves to a Christian origin, and that underlying religiosity in our cultures does inform the way in which we view the world. I concluded this when a friend pointed out to me the language we use in evolution

We describe evolved adaptations as serving a purpose. We'll say things like "we evolved opposable digits to better grasp things", and yeah, we all know that's not strictly true, but language informs our perspective and reflects it. We didn't evolve thumbs to hold things; We just got thumbs, and were able to hold things with them. These are not the same, and the former still has that kernel of creationism in it, some subconscious belief in a greater purpose

That said, I generally agree that an atheist might be made more militant if he had a particularly religious upbringing. Really, though, I suspect it's also a lot to do with insecurities. I grew up in a passively religious household, and was sent to a catholic extracurricular just so that I could choose for myself what to believe, and in that brief time, I actually became easily the most religious person in my house. Religion spoke to my insecurities and fears. I was bullied a lot at the time, and the thought that my righteousness would be rewarded and my bullies wickedness would be punished was wonderful. In turning atheist after that, it didn't undo the bullying. Instead, the self-righteous idea of "I'm smarter than you dumb Christians" was the new salve for insecurties

I'm way more tolerant now. Maybe the issue is just age. Maybe most of those awful ones are just obnoxious teens and young adults who would be obnoxious either way, and they'll grow out of it. If they don't, they get to become Ricky Gervais without the money or fame. Kinda rambled more than I meant to, but yeah, just throwing out some perspectives