Several studies, focused on the understanding of price volatility determinants in agricultural commodity markets, revealed that the joint influence of a plethora of causes is able to generate market instability. We investigate the contribution of endogenous and exogenous factors to global price volatility of four major grain (wheat, rice, corn, barley), adopting a Seemingly Unrelated Regression Equations model. We analyze global volatility, to conclude on short-run and long-run dynamics of markets instability. Our paper builds on existing literature by proposing a richer set of determinants of grain price volatility.