Well the reasoning's each CIV goes to war is probably blackboxed and handled individually for each CIV. If we consider if the game had an option for World War. Then prior to each AI taking its turn there would be a Declare-War-Check for each AI. If one AI is determined to declare war, then the other AI's would be re-checked.
Thus you enable a feature where on one turn the Romans might declare war on the player, one of your other neighbours might also declare war on you.
One of Romes neighbours might declare war on them to take advantage of Rome....etc.
Thus removing the BlackBox from war declarations, setting up whether a world war can occur, by doing a pre-turn DeclareWar logic loop.
[EDIT]
Also there could be a pre-war declaration, where the AI will contact you, and attempt to bribe you to stop it from joining the merryband of warmongers against you. It could offer to Sweden/Abstain, or perhaps join your defense...