The dog's hind legs suddenly become lame and one of the paws does not touch the ground. This may be caused by trauma, arthritis, interdigital inflammation, fractures, or nails that are too long. It is best to check the dog's hind legs for trauma and Check whether the nails are too long. If there is no trauma, you need to touch the dog's hind legs to see if there is any pain reaction, and then take the dog to the pet hospital to check whether there are fractures to avoid delaying the condition.