Rumors of foreign interference to end the revolution is the right answer.
Explanation:
It was rumored that Louis XVI, the King of France had entered into confidential colloquies with the rulers of Prussia and Austria to uproot the revolution. It infuriated the rebels and hence National Assembly announced war against the Prussia, Austria and against Louis XVI and his followers. Anyone suspicious of resisting the Revolution was guillotined and executed. Accordingly, the Revolution grew more and more vehement.
