I think trivial inconveniences are a huge part of it. I once was part of this group that created a website, and the owner put an edit button on the main page. If you clicked it you could see the source code of the entire website and modify any part of it.

I only have basic knowledge of HTML and javascript, but I ended up spending a bunch of time improving the site. I mean it wasn't a lot, I think I just did some minor editing and added a few pages that embedded an IRC client and an embedded version of our subreddit.

But to my memory, it's the only time I've contributed anything to any kind of open source project. And just because it was so trivial to jump in and get started.