From what I’ve been able to learn:
1953: Officially: meteor shower incident, Unofficially: that’s when giant metal pods fell out of the sky.
Also during this time the military set up shop, ⅓ of the Tumbleweed population died of some sort of contagion (otherwise known as water with lethal levels of lead in it). After that there have been various gas leaks every few years or so, varied reports of wild dogs taking to the streets once a month, gangs on PCP attacking every now and then, and the town undergoing quarantine or evacuation a few times over the years.
May 1962: town under quarantine for a month (said to be because of algae in the newly networked water supply)
January 1976: town evacuated for a week (train crash dumped toxic waste in the area that needed to be cleaned up)
October 1979: town evacuated for a week (toxic fumes coming from the new sewer system)
December 1985: town under quarantine (deadly strain of the flu)
September 1999: Local high school blown up by gas leak
December 1999: town evacuated again (truck crash outside of town dumped toxic waste into the streets and water supply). Was evacuated for two months. This corresponds with the attack on the base.
I’m not entirely sure what the real story is behind each of those incidents but if the military kept records (which its the military, they should have) there should be more information related to those incidents and what the actual threat was to the town at the time.