What causes an arrow to cork screw?

I have one arrow that flies in a cork screw or spiral pattern.

Looking at it, I can't see anything wrong with the shaft or the vanes.

It hits generally straight and will group with the rest of the arrows. It just flies funny.

I'm sure it's the arrow and not my bow because it only happens with this one arrow. It just needs to be disposed of and I'm not really worried about it but am curious as to what kind of defect would cause it. Is it not straight, spine miss-labeled, insert glued in wrong?

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Stand it on it's point on a flat surface and spin it in your hands like your trying to make a fire. I bet you'll feel some wobble in the shaft. I wouldn't hunt with it. Especially if you shoot fixed blade broadheads.

 

Are all your nocks indexed the same? Could be contact if that one is off of the others.

 

I never thought to double check the nock. I suppose if one vane is hitting on the thicker bristles at the bottom of the whisker biscuit then it could cause a wobble.

I really think it's in the shaft though.

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