Sometimes the fletching can give your eye the illusion the arrow is wobbling, especially if you're using a cock vane that's really contrasting with the background and hens that tend to blend in with the background. The arrow is spinning and that one vane is sticking out like a beacon while the other two fade into the scenery. So all you really see is the cock vane going round and round without the other two vanes showing up to balancewhat your eye is looking at,and it gives you the impression the tail is wobbling.

That's the first thing that comes to my mind, that you never noticed it before until shooting over snow. Might be your tune is off, I dunno. Just giving you something to think about before you go getting all worked up about your tuning.