This happens frequently because some users follow anyone and everyone, knowing that those they follow will probably follow them back. I try to. But then as soon as they rack up bigger numbers of followers, they start unfollowing so they have open slots to follow more, rack up bigger numbers, and keep repeating the process.
I never really pay attention to that because I'm mostly a casual Twitter user. I tweet about my friends' books and events, and I Tweets about what I'm doing. I get a kick out of the #FF hit parade. It's fun to give a shout-out to those I like and admire. Hitting the Twitter ceiling forced me to reassess my Twitter habit.
What To Do
The answer was to run Just Unfollow, an app that checks your Twitter account and lists your Unfollows so you can remove them. What an easy way to stay on top of the situation, which I hadn't realized was important to do.
Just Unfollow offers a free version and a premium version. The free version limits how many Unfollows you can remove each day. The app can also track those who recently unfollowed you so you can see who they are--and get the jump on deleting them I guess. It also can list those who are inactive Twitter followers.
I guess it pays to stay on top of your social network accounts since they're supposed to not only be for fun but also be a marketing asset.