Probably it is either aged rotted thread or poor quality thread that is just not holding up to the stress of being in the quilt. Old thread is weak thread. It will rot from sun and time just like any fabric will. Also I do not use Serger thread to piece with as it is just not as strong as regular thread if used singly, it is meant to be used in concert with two or three other threads in a totally different type of seaming. You don't have to spend a lot of money on thread though, the standard brands like Coats and Clark, Gutterman, etc. the kind you find in chain stores is good strong thread. If you sew a great deal and want to really save then go to thread on cones. I buy mine from Newark dressmaker supply, they have a web store. I get some cream, medium gray, and a dark, and that will do for most piecing, But I do buy colors too for machine quilting. Here is how to test your thread, take about an 18 length and get a firm hold on each end. Give it a sharp tug, sort of quick pop, if it breaks it is rotten. Now pull strongly on it, good thread should stretch a little but hold, cheap thread will not. I also invest in Bottom Line thread for the bobbin. It is more spendy but.....it is very fine and will go twice as long in a bobbin than regular thread, it is super strong, you must cut as it will not ordinarily break, the cones are very cost efficient. I buy a light Grey, almost a silver and use it for machine quilting. It just disappears into the back with very little pokies. Remember you are the most important tool in your sewing room and you labor is worth more than any other of the supplies you have. You deserve to have the best fabric and thread you can afford. Cheap thread is no bargain when your wonderful creations fall apart too soon.