As has mostly already been said, it’s really an ongoing debate of nature vs. nurture (vs. choice, in my own opinion). In general, it does seem that there are Zealots of every form. Most people have something that will get them very riled up. Sometimes it’s harmless, like very strong opinions on random issues, like raising children and eating habits, as YARNLADY said. Other times they manifest in ways that lead to trouble, and more obvious zealotry, like religion and sports (god, football riots, anyone? I’d say that’s pretty strong zealotry).
So, the main reason it happens across all religions is that it happens across almost every person. Some people can be apathetic about everything, but it’s quite rare to find someone who doesn’t have an unnaturally strong opinion about something, just takes some time to find out what that something is. The usual case is that it’s just a matter of how it manifests. If it’s a typical social thing, you mostly wouldn’t notice it as zealotry, just someone with a very strong opinion.
But, it seems that when this is echoed very strongly in other people, it tends to amplify itself (which happens frequently in social experiments). In sports, you get people yelling and screaming and flipping cars. In religion, you get people setting themselves on fire and blowing themselves and others up. It just happens to be easier to notice.