I didn’t turn dogmatic with respect to religion, and an atheist can give simple and simplistic answers. “is there a god?” “No.” Nuances can come later.
I wonder if some of it is from peer pressure, or family pressure. You might not go to church, or even care much, but Sunday school is so expected that someone might feel they’d have to join for the “good of the kids.” (Bad of the kids in my book.) Lots of so-called agnostics lack belief without good reasons, and might find it easier to go along with the pressure.
As for other types of dogmatism, since kids do better with solid rules and guidelines (like bed time) enforcing these tends to make one a bit more dogmatic, I think.