I would imagine that most bases were over run. Some even having specifically gathered specimens. That many people had been infected and tried to make a last ditch run for it to anywhere not where they were.
Turned in the air and crashed. Perhaps some one sniped them with all that military gear they found. Possibly someone who was not a pilot at all tried to make a run for it and took off into the air only to forget it was a gas eater and fell out of the sky.
Lotta possible scenarios =D