Bug or backstab? I ask neighbour for war pact. He immdtly declares war on me instead.

[Maniac]

I've been gifted the Polynesians as a birthday present. So now I'm very busy losing the Paradise Found scenario. I need to make a last ditch attempt to conquer Tonga before they win a cultural victory. For that I need to cross half the Pacific, so I'd like to keep my direct neighbour off my back while I'm defenseless at home. I figured a good way to do that would be to bribe my neighbour to go to war with Tonga as well. Now the strange thing is that when I bribe him to declare war against Tonga, he declares war against me instead... Is this some bug, or in fact the perfect backstab?? It's the first time in Civ5 I've felt the need to bribe someone to go to war, so this is new territory for me.

To clarify, I know backstabbing in general is a common occurence in Civ5 - I'm asking people who have experience with asking other civs to go to war with someone, if an immediate backstab then, while still in the diplomacy screen, is part of the game.

If this isn't a bug, Firaxis should really add some diplomatic message "Haha! I just backstabbed you! " to avoid confusion.

Screenshots attached.

[Bob_]

Are you sure that you are not Tonga yourself?
Perhaps there is some mistake in the scenario itself so that names are confused somehow.