The quilter probably used a nylon thread, a common practice for a while in the early 1990s. Most quilters stopped doing this when they began to have some of the same problems you're noticing. The thread manufacturers began to make a softer nylon thread, but still many quilters had decided to forego using nylon threads by then.
Next time, be sure that your longarmer is using good quality cotton thread. As a matter of fact, if it were me, I'd be using a different quilter, LOL!
Jan in VA