Certain leaders can be bribed into going to war. The further behind they are on technology (or gold?) the higher the likelihood of your opponent being able to drop a fat stack on your friend to backstab you. Try not to let your friends fall too far behind. If you are the tech leader, keeping your friends close keeps them from going to war voluntarily and also keeps rivals from having the resources to force them to betray you (double benefits since AIs ask for higher prices to betray closer friends).
AIs don't attack if they aren't prepared for war. I can bribe Alexander all I want and he will start cranking out the units, but until he is good and ready, he'll broker peace ASAP and take my techs without batting an eye. However, once he DOES have an army, he will often declare war voluntarily on his weakest unfriendly rival. Make sure that you are friends with the strongest and pick on the weakest. Just like in school, ya know?
Also, at any level, you'll find that its a bad idea to have smallest army or to lose at all. Once you start losing, winning is usually not going to happen. This is what I call "kneecapping." If I kneecap my opponent, he won't get up. He might get a one stack army one day that might threaten a border city of mine, but basically, if I focused my full weight on obliterating him, he's smoked. AI thinks the same thing about humans. If you have a small army, few cities, no resources, and aren't defensive packed with much bigger and stronger civs, you will be the target of choice. Dogpiling is not uncommon either. I have bribed AIs to war for free when the nation I'm picking on is small and I already have an ally or two. Don't be that weak civilization. I always check Demographics to make sure my military is in or near the top 3 within a reasonable deviation. If someone has three times the army that you have, you should probably quit the game unless you are playing Always Peace, someone is going to eat you.
). I would gather from what you have said, that it was probably a randomized event. 
