Probably extreme amount of shock. Just staying there without going into flight/fight mode is actually a lot more common in the animal world than you'd expect, including humans.
That dog appears upset that his master hasn't left, keeping him from doing what he really wants to do for his birthday, hanging out with his buddies.
Huh. I was thinking more like "this is cool, but I don't know what to do in this situation. Did you say there was cake?"
Awwww, Keeva looks so sad, like she knows she did something bad and is waiting to be punished... It isn't your fault, sad doggy....