Is this the thread you are referring to?
http://www.quiltingboard.com/main-f1...e-t238138.html
If so, I don't think deterioration of the thread is the culprit. The thread would have to already be deteriorated before being sewn (dry rot) to account for this kind of breakage. It's good to keep in mind that the farther apart quilting lines are, the more stress will be put on the thread when the quilt is used or washed. Also, it's a good idea when using a top loading washer to never actually let the machine agitate the quilt. Rule-of-thumb is to stop the washer, agitate by hand (pushing down on the quilt), then advance to spin for both wash and rinse cycles.
If you think about it, hanging a quilt on a clothesline puts tremendous stress on the quilting lines; water is very heavy, and the added water weight in the quilt can cause thread to snap. The same thing can happen in a top-loading washing machine with the typical central post agitator. Thinking back to when I washed clothes in a top-loader, there were many times when the clothes came out all twisted (and sometimes knotted together). This wasn't so much of a problem with small loads (and may not affect smaller quilts, such as those for kids) because they had space to slosh around in the water. A large quilt stuffed into a top-loading machine could get twisted enough to put tremendous stress on quilting lines, especially if the quilting lines are far apart. (The further apart the quilting lines, the more stress on each quilting line.)
I believed in the "break" test for thread too, until I went today and tried out threads I have successfully used in quilts that have been washed and dried. Both the Signature and Glide threads broke easily when I jerked my hands. I think a bigger factor is how close together the quilting lines are and, of course, whether machine agitation in a top-loader was used.