Mine hasn't but what is the main cause of string breakage? Does it happen often? How often should one change the string or cables?

Doing a search would get you lots of answers quicker but...
The only one I've had break on me was because I drilled out the peep with it on my bow (many years ago). About the time the aluminum peep was getting warm in my fingers a lightbulb came on in my head but it was too late. So you could say physical abuse as one (also rubbing up against brush, etc.), failure to maintain the string (waxing- when I helped at a shop we'd get alot right before hunting season that broke under the serving right at the nock-usually these we founf out were 4-5 years old and never saw wax), shooting too light an arrow as probrably the main ones. I make my own and change them every year. Most of the concensus I believe is to chang the string every year and the cable(s) every other year. This depends on the amount of shooting you do of course.

I realy can't add anything more to what was said above. I like to replace my strings and cables every year. Why take chances?

TAKE YOUR KIDS HUNTING AND YOU WON'T BE HUNTING FOR YOUR KIDS

I would agree with both of the above posts but would also add that besides poor maintenance and normal wear and tear on the bowstring there is also the issue of shooting too light of an arrow. A bow's string is usually the first thing to go if you continually shoot arrows that are too light for your setup.