I think it would be the progression from gathering firewood in the winter. They would notice that logs are easier to drag in the snow/on ice. Logs also glide way more easily along their length as opposed to across the side. So maybe you pick a couple long logs to pile all your other sticks on so you can drag a whole heap of wood in one go. Eventually that gets refined into a sleigh with bone runners. Then you just need someone who wants to have some fun with the idea and plays around with making a miniature version they can strap to their feet.