I think you've just proved the point about having 4 snow tires at all the wheels. There was more traction at the front of the car than the back, so you end up fishtailing when there's less traction in the rear.
Anyway, I'm not sure if other people already mentioned it, but in winter driving, the key is no sudden moves and take it slow on the turns. Hope your ABS is in good working condition, I feel mine kick in once in a while when I'm not expecting it. Watch out for black ice, you should roll up to stop signs and lights, not brake at the last minute.
