Depending on the breed of the puppy - it could be a hereditary trait. I have Australian Shepherds, and have also had Rottweilers. Occasionaly in a litter in those breeds a pup will be born with what is called a natural bob tail (NBT). There's nothing wrong with this occurring - just natures way.
Though if it does concern you, or your pup seems to be straining when going to the bathroom - you may want to have your vet check the puppy out to be on the safe side.
Though if it does concern you, or your pup seems to be straining when going to the bathroom - you may want to have your vet check the puppy out to be on the safe side.