If you search for the character '+' in the list of e-mails you can get an idea where the mails leaked from. It seems to me like this is a collection of databases scraped from different sources as others have suggested. For Gmail users, it's a good practice to register to websites using username+websitename@gmail.com (e.g. mark.samman+hackernews@gmail.com), that way you'll know who leaked your data when it appears in…
If I were a bad behaved site and sell (sold? sorry, tired and non native speaker) my userlist, i'd probably remove all plus-suffixes from the addresses. Same as if I were a blackhat stealing them to be sold.
Honest question, no snark.