RE: What do you think would cause this
Salt,
The guys have said a mouthfull. I just want to add that your arrows may indeed be fishtailing, but that won't stop them from hitting where your bow is sighted in to shoot them. And they will probably group well, too. That's because they are all doing the same thing and so is the bow. And this is with field tips in. You put some fixed blade broadheads on them and things could get real ugly (little wings on the front). That's why tuning for good arrow flight is important. Good luck.