I once had something crazy like that happen. There was apparently a complicated system of alliances and there was a pretty big arms race going on between the Korath and the Iconians. Eventually a disgruntled miner from the Iso Congregation (custom race) shot the leader of the Korath, so the Korath delcared war. The Iconians were allied with the Iso Congregation, so they delcared war back, and from then on it was like dominoes. The Altarians came to my aide because we shared a similar ethical alignment, but then they declared war on me because they had an alliance with the Torians, who had declared war on me because they had an alliance with the Iconians, who I had declared war on because I had an alliance with the Agrarian Empire, who were allied with some other civ... yeah, it was a big mess. It was really chaotic for about a decade after all that, but then it was just the Agrarian Empire left. It was good stuff.