It feels good to get a mail whenever someone follows you over Twitter but losing a follower is not so happening event. Even worse, you can not figure out that who stopped following you (in case you have a good number followers).
To solve this problem, you can now use Qwitter to track people who stopped following you. The application requires your Twitter handle and an email address to send you a weekly email showing all your Twitter unfollowers (see below pic)
Now, you can find out that which Tweets made you lose followers. This is a Free application so go ahead and make best use of it.