Hoppers and other large flies can spin resulting in a twisted line. It usually happens at the end of the drift when the fly is dragging in the downstream current. If it happens more than once, I change flies. It seems some flies are tied so as to be unbalanced resulting in spinning under pressure. I use 5x and 6x for trout and have been using Orvis tippet - mostly because I live close to their store and its convenient to stick with one brand so the spools connect to each other.