Rod is definitely right about the stiffness of hackles causing twist; but I'd say it was due also to leaving open turns of hackle, producing an 'airscrew' effect; because it can happen with softer cock hackles, too, especially when using soft, thin tippet material. When Orvis D/S first came out, I gave up using their 2and 3/4 pound material, because of twisting. That was a while back.
Also, the Griffiths Gnat is a palmered fly, I believe, although I dont use it, and having open turns down the body makes it a pretty good airscrew, I'd say.
Line twist and leader twist can occur as a result of 'circling' the rod tip, by taking the back cast out to the side, then making the fore cast straight overhead, to avoid the fly hitting the line or leader, so, I presume, could excessive roll-casting. I'm sure Phil is right, too, about mis-matched fly size to tippet weight. TC
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