Snowbanks tend to build layer at a time - once a storm comes, a snowbank is created, and can be added to with each additional plowing, if the first storm is a significant one, and then with each new storm. The cats could have settled down in a spot in the snowbank, and, probably being weak already, just curled together and fell asleep, and were sounds asleep when the next plow cam. You hear the plow coming for quite a while before it gets to your spot, and maybe they, sleeping were just now awar of how close it was getting. I doubt anyone could have dug a hole and stuffed 5 cats in it, if said cats were awake! You know how hard it is to persuade ONE cat to do something against its will ...

In any case, thanks to that dog, they were found, and are safe now, which is where we should be focusing our attention.