Interesting topic. In fact this same subject came up on another board recently. It seems that the cause was thought to be related to abrasion caused by an imperfect finish at the eye of the hook. I've not personally experienced this phenomenon although it would make sense. I'm a great believer in loop knots for fly connection for anything other than droppers or dry flies. I also try to remember to change leaders after a prolonged fight with a large fish and always start the day with a whole new setup. I figure a couple of bucks worth of mono to be a fair price to pay for eliminating one possible cause of losing the fish of a lifetime.