I have never had this happen, espeically not with a cotton bat. However, I have never HQ using W&N. I machine quilt with it all the time. I usually like to use a poly for handquilting it is easier to needle but I have also used 80/20 blends. I have had bearding issues with polys in the past. You should be using a quilting between needle. It is finer and smaller. But I would think if this is only happening on the back and not the front your needle isn't the problem. What are you using for backing? It could be the fabric is too loosely woven?
Your thread is fine, with thread just use what you like to work with. I do run my thread through bees wax but that is to keep it from tangling and knotting on me.
I guess I am not much help. This problem strikes me as very unusual but maybe one of the other hand quilters will have a good suggestion.