Can someone clarify something for me?

I am currently playing as Yor, and second most powerful civilization under the Drengin lords. We have had impecable relations, and all of a sudden they declare war on me.

As I choose to speak to the Drengin, they are still listed as having the closest relations to me, and no "war" line is drawn between us in the foreign relations screen. Further, I cannot negotiate peace with them.

They continue to attack me with ships that are superiour by far and yet they are completely cordial when I wish to speak to them.

Is this a bug or is this suppossed to happen?