[Mr Twisty]

The ABS works even at 2mph... the thing that makes it strange, ABS doesn't know how to feather the brakes, it only knows "full on" and "full off" and does it very quickly. When you're going slow on ice it feels like the brakes are malfunctioning.

Having said that, yes your front brakes could possibly have been locked up, but then you wouldn't be able to steer. If it's FWD, the backs could be frozen but the car would be tail happy, like a dog about to get a treat.

I would venture a guess, the only thing that changed from your past experiences, this time it was so slick you could hardly stand up on it. When it's super cold you can get a slight amount of grip on ice, when it's close to 32deg or the sun is out, and there's a bit of water on top of the ice, that's when things get crazy slick.

edit: I re-read your post, you may have had a brake or two that was frozen in the open position, or ice in-between the pad and rotor, and when you braked it cracked up the ice.