Albert Barnes on Acts 19:40

To be called in question. By the government; by the Roman authority. Such a tumult, continued for so long a time, would be likely to attract the attention of the magistrates, and expose them to their displeasure. Popular commotions were justly dreaded by the Roman government; and such an assembly as this, convened without any good cause, would not escape their notice. There was a Roman law which made it capital for any one to be engaged in promoting a riot. Qui caetum, et concursum fecerit, capite puniatur: "He who raises a mob, let him be punished with death."