Puffing and darkening beards is a sign of dominance; your beardie is trying to tell you that s/he is king of the house. Opening the mouth can mean 2 things; that your beardie is thermoregulating (s/he is trying to cool off, nothing to worry about) or it means that s/he is showing you all those pointy little teeth as a warning. They may also hiss.
These signs of aggression, as seven65 said, could be hormones. We are rapidly approaching breeding season. Maybe your dragon just wants a mate. Or, if there is a loud noise or fast movement that startled your beardie, s/he may put up defenses. Either way, it's nothing to worry about, most beardies puff a bit at some point in their lives, some more than others.