If they were migratory they wouldn’t need to be driven, now would they?
Migratory if nothing were in there way...no fences...they don’t gather themselves up and neatly move, even if gates were left open few would find the gate- they would drift to a fence and stand there and freeze to death, the snow would pile up around them while they are moving trying to find a way out- making another type of fence to trap them. Cows are somewhat migratory in storms, but have no sense that their nature may be their worst enemy. Their nature would be to head away from the storm, when due to fences they would be smarter to hunker down here and there with whatever shelter they can find in the way of terrain. Instinct is hard to overcome. Even without all the fences there are today, sudden catastrophic snow storms have killed many cattle, huge losses during the late 1800s.
I have had experience with cattle in a catastrophic snow storm and if the rancher my dad worked for had not been large enough to have a large crew of cowboys, able to rent a helicopter (to spot all the odd groups of cattle trapped all over, and to drop hay from), had a Caterpillar to break trails to drive cattle to where they could be fed (and lucky enough the storm cleared enough to use it all soon enough to save most of the cattle) there would have been a huge death loss. As it was there was not enough workers, feed, or other supplies to do a great job, but most were saved.