Hi, me again, yes I'm everywhere,
Without realising it you are all encouraging your dogs to bark as you leave, simply because it works. Pack animals need to communicate with each other and barking, "You're going to leave, you're going to leave!" is rewarded because you do! Herding breeds have a head start, they know when you are leaving and see it as their duty to show the way, as they would with sheep, from behind!
Sitting the dog and rewarding with food when a quiet response is given is the perfect answer.