COD--Brown sugar snow does that to almost every wheeled vehicle. It's not limited just to bikes, although the consequences are greater for us.
As for technique, I had less trouble with it last year on my hybrid than I do this year with the Portland. (Using the very same set of tires on both bikes.) It seems sensitive to the front/rear weight distribution. I have more problems with the road bike in brown sugar snow because more weight is on the front. If I shift back, it's better. Full panniers and it's better still.
Hang a few bricks off your rack and see if that helps.
Oh, reduced tire pressure helps too. I've run my 700x35 Nokian W106 down as low as 25psi in the front.