Since plebeians when summoned as soldiers had no way to support their families, yet couldn't avoid military service, some of them resolved to leave and found a new colony. To persuade them to stay, the patricians offered them a salvo -- political representatives called tribunes with veto power. The plebeians accepted.
Many patricians believed they'd lost too much in the deal, so when an opportunity (famine) presented itself to reduce plebeian clout, Coriolanus leapt on it.
Rome imported grain to dole out to the suffering people. Before it could be administered, Coriolanus said publicly that the plebs must choose between corn and tribunes.
Reaction was so furious, Coriolanus fled to the Volscians. They were pleased to have a competent general to lead them against Rome. And Coriolanus would have done so, had not his mother (Veturia), wife (Volumnia), and children come to stop him.
On the brink of battle, at his mother's request, Coriolanus put down his arms, turned back, and lived out his life in self-imposed exile.
