Magnus
Diplocat
This has happened enough times that I think it can be called a definite AI tactic.
Basically, when I am on top of the powergraph and all other civs are uncooperative at best, and generally hostile, suddenly out of the blue a few of them become 'receptive'. But this only means they are going to backstab you within a few turns - it never fails! and not surprisingly, if you think maybe you can get something from this new 'receptive' civ, think again, they almost immediately go to war with you over the most trivial thing - but I suppose its better to go to war than be backstabbed. So keep a vigil on your foreign advisor - that will help you to ward off treachery!
Basically, when I am on top of the powergraph and all other civs are uncooperative at best, and generally hostile, suddenly out of the blue a few of them become 'receptive'. But this only means they are going to backstab you within a few turns - it never fails! and not surprisingly, if you think maybe you can get something from this new 'receptive' civ, think again, they almost immediately go to war with you over the most trivial thing - but I suppose its better to go to war than be backstabbed. So keep a vigil on your foreign advisor - that will help you to ward off treachery!