Have you asked yourself this? Well you're in luck - ManageFlitter has the answer!
When you upgrade to a ManageFlitter Pro plan, we'll start tracking who's unfollowed you. If you feel like it, you can then unfollow them quickly.
Apart from being the perfect answer to who unfollowed me, ManageFlitter is a world class Twitter management tool. ManageFlitter's feature focus on 5 key areas of Twitter; Unfollowing, Following, Searching, Twitter Analytics and Posting.
More detail about each of these can be found on the ManageFlitter Features page.
It costs $12 per month to start tracking who has unfollowed you. That's only $2.80 per week! ManageFlitter also contains many more powerful features. While ManageFlitter is very usable in the free version, many agencies & individuals sign up to the Pro service for much faster growth of their Twitter account.