I agree with Lori S. My first thought was if your needle size (eye size) was just a bit too small for the thread, the eye could have caused 'chaffing' of the thread which weakened it - especially if the thread was not a long staple thread (which I don't believe C&C is).
And again agree with Lori ... could be the length of the stitch if thread was pulled too tight while quilting. We're these quilts FMQ'd or quilted with a walking foot? The tension could also have been a little too tight.
Combination of all of the above - tension, wrong needle size, and stitched too taut.
If I were you, I would start by A) switching to a different thread, either a long staple cotton or a cotton wrapped poly. B) making sure you have the right needle size for the thread you are using and the task you are performing. You can find some GREAT info online about threads and needles - and their correlation to each other, often on the manufacturers web site. Check out superiorthreads.com for lots and lots of "threaducation" including needles. And while you're there - check out the thread they sell too - awesome stuff.