Another thought is that it is fairly mild at the minute aswell so that could be a contributing factor.
It's definitely this. I use Goodyear Ultragrip and although they are good for sedate driving, if your pushing on, they are no match for decent summer tyre at the right temperature.
I've put my Summers back on the front until it snows (Have 8 monza wheels, so easy to swap about)
Left the winters on the rear, and will put the other winters on the front when it snows or pro-longed <5*C tempeartures hit.