Some people enjoy the Blogger pastime of Following so enthusiastically, that they try to Follow every interesting blog that they see. Some of these enthusiasts have not yet learned that there is a limit here - you may Follow no more than 300 blogs.
No doubt, they find out about the limit by direct experience.
For a while, having reached the limit of 300, they will Follow no more new blogs - just stop at 300. Eventually, though, they will see yet one more blog that really interests them - that they "just have to Follow". In order to do this, they will have to discontinue Following one of the 300 blogs that they are currently Following.
People who watch their Followed numbers avidly will occasionally see their Followers count go down. Many people who publish blogs, and have audiences who routinely Follow the maximum of 300 blogs, will see this happen more frequently. And the more Followers you have, if your blog is in this category, the greater the chance that your blog will lose one Follower who has to drop your blog in order to Follow another.
This is an effect similar to the churn in search engine hit list position. Just keep publishing, and the count will go up, again.