We already covered one way to find out who unfollows you on Twitter, but here is another way which also tells you why (your last tweet before they unfollowed you): Quitter. You only have to enter your Twitter username, an email address to contact you with, and you’ll recieve updates like this:
Next time you say something unintentionally offensive, or reach a point where your noise is to high to your signal, you’ll have some earlier and more detailed warnings, as opposed to a simply disappointing and disappearing follow number.