The emperor [i.e., Emperor Yang's father Emperor Wen] deposed his proper heir [i.e., Emperor Yang's older brother Yang Yong] and gave the throne to this current emperor, and this led to the current troubles. The Duke of Tang is an honored relative of the imperial clan, and he did not dare to sit by watching for the state to fail, and therefore he rose in righteousness, wanting to depose the improper emperor.
Emperor Wen's decision to remove his legitimate successor, Emperor Yang's older brother Yang Yong, and install himself as emperor set off a chain reaction that led to the present troubles. The Duke of Tang, a revered ancestor of the imperial family, could not stand by while the state faced imminent collapse, so he took bold action in righteousness, determined to overthrow the usurper.