People driving far too fast for conditions and tailgating is the primary cause. Too many just see "70" on the sign and believe that they have to drive 70. There is no need for them to pay attention to conditions. Then, the "stuff" hits the fan, and you have the real recipe for disaster.
It's winter, it's Michigan. It is not a secret that salt is not effective below certain temps. It is not a secret that the western side of the state is subject to lake effect snow. It is known that, as the road conditions are "less than perfect", one needs to slow down. Too many believe that road conditions, and physics, for some reason, don't apply to them.