This is called "bearding" and can be caused by any of several issues.
1. The thread is waxed or has a finish that attracts the fibers of the batting. "Glazed" cotton hand quilting threads are particularly guilty of this, but other threads do it too.
2. Polyester battings have done this frequently in the past; as I have not used polyester for many years, it's possible I don't know if this issue has changed. Yet, I've had cottons and even wool beard if...
3. I have used a larger needle than necessary for the threads. As I have arthritis in my hands, I frequently use a larger needle so I have better control. But this makes a hole just big enough for those batting fibers to migrate through the top or backing.
There may be other reasons, washing or not washing a top or batting may affect bearding, but the above ones are those that I've noticed over the years.
Jan in VA