Why does my puppy barks when i move my legs while sitting on a chair?

i have a 3 months old gull terr. He is very loving and social , though the breed is famous for its strength and territorial nature. Gull terr's are notorious for even killing strangers in their territory. Mine is very sweet but he barks at my legs movement when i sit on a chair placed right infront of him. Is he scared? He barks and eventually lays flat on ground . Barking never stops untill i get up or stop moving my legs.