You Twitter followers can unfollow you for different reasons.
Most people don’t really care when others unfollow them on Twiter but if you belong to the other category and want to track every single person or brand or spammers who have stopped following you on on Twitter , here are a few services to help you out:
With NuthshellMail, you can track new followers and unfollowers across all your Twitter accounts from a central place (your email inbox) and since the service uses OAuth, you don’t have to share your Twitter credentials anywhere.
Also see: Why Twitter Users Need NutShellMail
2. Goodbye Buddy – Follow @goodbyebuddy on Twitter and this bot will start watch your Twitter followers. You can log into their website to see who unfollowed you and when – you may also unfollow them back and even block or unblock them.
* Qwitter is another service that sends you an email as soon as you lose a follower on twitter but I skipped it as I am not too sure if the service works anymore though their website is very much functional.
For more tips and tricks, check The Twitter Guide.