AI aggression is on on max... in this case 3 factions have been destroyed by Nordheim previously, but all surviving factions have been persuaded in a friendly fashion to vote for their new overlord accordingly...
I've had this once before with the Palaceans, but with them a more severe case of peace broke out... all the factions remained at peace with the player and vice versa, untill an election victory decided the outcome which was 100% in favour of Palaceans ruling the galaxy...
I think its simple the case of excessive diplomacy bonuses, overwhelming monsters/fleet and a large number of player worlds affecting the result, that and none of the others trusted each other, its by no means normal... Lol
Its usually very easy to become involved in a war with the other factions on max aggression, you need only leave a small ship in the middle of an enemy fleet... at least you get them to take the blame...