The sign is there for a reason. The Woodstock Police Department posted this on Facebook yesterday with the message, "During the snow, you'll see signs on plow trucks that say "Keep Back"
The reason for this is illustrated in the photo. This plow struck a large pothole, causing the plow to dig in. The momentum of the truck caused it to override the plow and come to a sudden stop. Any vehicles that would be following too close would have been involved in a rather serious crash. Luckily, no one was injured."
Check out the picture below.