I guess some common ones are,

1. sys-admin reorganisation, moving content from one spot to another without redirects in place.

2. developer reorganisation, for example moving from "confusing" urls to "slug" based urls without adding redirects

3. fragile content, content that moves depending on external changes (beta to release for example)

4. product retiring or companies getting acquired

5. hackers messing stuff up in a way that can not be fully repaired (or an non-recoverable data loss)

1. people get tired of hosting blogs on dreamhost

2. list mirrors like nabble do wholesale migrations without redirects (Google groups is gong thru this now with new format (but with redirects, I hope):

    groups.google.com/forum/#!msg

3. wiki's get pages duped and branched to where their marginal utility is 0, so the sponsor decides to start over.

Just a guess, but I'd think the most common cause of broken links is the original administrator retiring that whole branch of the site (or the site itself). Often the domain still works but that old stuff has been thrown away.

Often it's clear that what's there now is a lot better or more professional, so you can see why the person didn't feel like messing up the site with the old stuff - but that old stuff is still gone.

Other times the domain is gone, since the person or people have moved on to doing a lot better stuff and stopped maintaining that old site - "why bother." People change - a web site isn't something you publish once, it's something you publish every time your server answers an http request. Would you keep publishing everything you wrote 10 years ago?