Bait dogs are animals who are cruelly used as "practice targets" in dog fights. People will encourage fighting dogs to attack the bait dog, while making sure the bait dog is unable to fight back. To do this, they might tie the bait dog to a tree or pole, tape their mouth shut or file down their teeth.
"Dogs with teeth filed down or extracted could be an indication that the animal is used as bait to minimize injuries to fighting dogs during training," a spokesperson for the ASPCA said in a statement.
Another reason for teeth filing is related to breeding - People will file down female dogs' teeth to make it easier for male dogs to breed with them. But since Duke was a male himself, being used as a bait dog is the most likely explanation.