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 lists like this or when you get spam. Gmail ignores the plus character and anything that follows the plus. You can also add dots at arbitrary positions in the username part.