What would annoy me about it at least, is the implication that whatever you’re currently doing doesn’t matter at all. To him, whatever is currently on his mind takes precedence over anything you could possibly be doing at the same time. He doesn’t even think about how important or relevant to you the thing is that he wants to show you, because he doesn’t value your time the same as his. You’re supposed to just drop everything and do as you’re told.
Not telling you what it’s about also increases the sense of urgency (the implication being that there’s no time to explain; the bird could be gone from the windowsill or the kid could have fallen out of the window) while at the same time making it harder to make an informed decision about actually hurrying over or refusing. If you’d ask for an explanation, it might already be too late after all.