The U.S. Air Force is moving its aircraft from Elmendorf Air Force Base in Alaska, to escape an eruption by a volcano (Mount Redoubt). Volcanoes put out lots of gritty ash, which damages or destroys jet engines. A coating of ash, not to mention flying rocks and lava, is also bad for aircraft, and the equipment used to maintain them.
Volcanoes are not the only natural disaster that the air force flies away from. Every year, during the late Summer, early Fall hurricane season in the western Atlantic, the U.S. Air Force makes plans for where they will move aircraft that find their U.S. bases that are in the path of a storm. A favorite refuge is Wright-Patterson Air Force Base, which is large, and in the middle of the country (western Ohio). Last year, as Hurricane Hanna (which had calmed down to tropical storm status) moved up the east coast of North America, military and commercial aircraft began to flee. In the days before the storm was scheduled to come ashore, 14 F-15 fighters flew into Wright-Patterson from Florida, and another 80 from North Carolina. In addition to the combat aircraft, tankers, transports and anything else that can get blown around, and is able to fly, is sent off to a safer place. The USAF has similar evacuation plans for aircraft in the Pacific, where even more powerful storms (typhoons) are a regular occurrence.
The navy and army will also fly off their aircraft and helicopters to safer locations, as do commercial aviation companies. It's expensive to do all this moving around, but a lot cheaper than risking aircraft being blown over and sustaining millions of dollars in damage. There are frequent examples of this, as there are always some aircraft that are being repaired, and not flyable. These are tied down or otherwise secured, but still tend to suffer damage if they a directly in the way of the storm.