My mom has a maltese dog (maybe 7 years old) that is mostly well behaved. Except when he hears someone leave to go downstairs or out the front door, he barks at that person quite vigorously. i could be in the kitchen doing stuff and he's way over in the living room and hears me open the staircase door to go downstairs; he comes charging and barking barking barking. occasionally barking so vigorously that he bites me on the ankle a little.

I have no clue why he does this. does he not want people to leave out of separation anxiety? is it a defense mechanism? I really don't know much about dogs. does anyone know why and how to stop this behavior?