3 Reasons that I know for why websites lose their page rank:
1)The backlinks that were looking to you removed their link
2)Your site has too many outbound links (you are linking to too many other sites, so you lose the link power)
3)A few websites that you are linking to have become banned, and it's damaging your site's rep due to the linking to their site.
There's other reasons as well, but this is pretty much the main reasons I've seen with my own sites and others.