Maybe in small part at the beginning, but...
I was involved in one of these in Minnesota years ago...smaller, but same cause. When the people at the front of the herd suddenly lose sight of the cars ahead and the road around them they do the logical thing...they slow down. The folks coming up from the rear don't recognize the loss of visibility ahead and they can't see the slowing traffic so they just drive headlong into the growing mess. If the road is slippery, they can't steer and the can't stop, so even if they realize what's happening in front of them they're going way too fast to avoid it.