There is no official way to find about the user who has unfollowed you on Twitter. However, there are services which are unofficially available and allows you to see which user has unfollowed you. These services typically require you to granting access to your Twitter profile, before you see any results.
Below are three popular services which allow you to see the user who has unfollowed you:
There are a few more services that can handle the task. However, the trust level of these sites was more above those. I personally didn’t like the experience that the third party sites bought. At few places, I found it more annoying than helpful.
It would make sense for Twitter to take-up the analytics issue themselves, and provide some sort of way around for users. They already do a good job, but it’s hardly useful in detail level.