Here are several possible causes for a swollen leg in dogs. Trauma to the Leg. If your pet has recently had any type of accident or fall, the traumatic injury could cause your pet's leg to swell. When you examine the leg, it should be relatively easy to find the location of the trauma. While checking your dog's leg, if she begins to moan, cry, struggle or snap at you, stop what you're doing and visit a veterinarian as soon as possible.