Originally Posted by Ricky
This used to happen to me a lot. I found that I was " crowding my work". Meaning that I had the trimmer to close to the object, causing the string to bend when it was against it. When the string clears the object it straightens out again. When this happens over and over it will break the string (or melt it sometimes, sticking to the other string inside, depending on the brand). Just get the string close enough to cut the grass and no more. You need to do this against trees and other plants to avoid damage. Try slowing the rpm's also.
Since there's no visual aid (such as video) to further illustrate what exactly is happening, I'd have to agree with Ricky on this. It's a common flaw in some people's edging/trimming technique that can cause lines to break rapidly.