Yeah, you're right, Johnstown gets hit with flash floods regularly because of its unfortunate punchbowl geography and the confluence of its two rivers. The "shoot to kill" order was first issued by the local police in 1889 when (false) rumors spread of widespread rapine and pillage the day after the big flood. Undoubtedly, the order remained a solid tradition in subsequent floods.
I just finished reading McCullogh's classic book about the Johnstown Flood, and what is remarkable is how the community came together, reformed its government and launched search and rescue and rebuilding efforts on its own within a day or two, without federal of state help, which did not arrive for nearly a week after the disaster.