Because the Jacobins in control absolutely believed the Austrians (and co.) were out to get the Revolution following the Declaration of Pillnitz (1791). While this was never the idea behind the Habsburg declaration (it was an empty gesture to somehow protect the royal family), it served as a red flag to the hawks in the Convention. One of the better examples of a self-fulfilling prophecy: everyone is out to get us > let's go out and get everyone!! > mon dieu: everyone is out to get us!!!