I had this issue with a quilt top that just started coming apart for what seemed like no apparent reason. Well .. come to find out my iron was over heating and the thread became brittle. I had used a poly cotton for the piecing. I had not realized what was happening withthe iron until the problem became extreme and I saw the thread literally disappear before my eyes as I was pressing a block on the next project. Then I knew what was happening with the quilt I had just completed that different parts just seemed to have the thread vanish.
Needless to say that iron was in the trash so fast.