Snow can be very heavy depending on the water content. So sometimes it's really light and basically effortless to shovel, but sometimes each shovel full of snow is 10-20 pounds of weight that you have to throw over and over. That gets taxing very quickly, plus people don't generally warm up before doing this intense exercise.

It's not like shoveling snow is super dangerous. Most people don't die from shoveling the snow. But it can happen and it's worth slowing down and taking breaks.