You just pulled out your string trimmer after a long winter's nap. It starts up fine, and at first glance the string looks okay.
Just as you begin trimming, you start to notice small pieces of the line breaking off. As you continue to trim, your line's getting shorter.
String trimmer line gets quite brittle with age, which causes it to break off in small pieces as you use it. Replacing your string will correct this problem and provide better efficiency.