I have an un-scientific theory about that because I have on many occasions had the same thing happen to me. I think the fish that go after the knot think it is a food source. Being competitive behavior or instinct(or what ever you call it), the fish thinks it is stealing the meal from the smaller fish(Fly ). I've also experimented with a 2 fly set up because of that. Although now I coat my knot with "UV Knot Sence", now I have less drag and wake(do to a bear knot) that the fish goes for my fly now.