I've seen this instant peeing come from fear mostly. That's why I asked. When I worked for a vet years ago some dogs would pee as soon as they walked in the door or saw the vet. And when I worked with a groomer that was discovered abusing animals, the dogs would pee and poop as soon as they saw her, no one else. They say dogs have a sixth sense when it comes to telling who's naughty or nice.