It was kids....
My guess would be the clips were on only one side of the waist allowing the missing pants to fill like a spiniker and get pulled of by the wind.
Did a cold front blow through your area? Sometimes when the weather changes the wind will blow in one direction then suddenly shift and blow from another. Almost reverses itself. When it does this it can create a swirling action.
I don't think a tornado blew through your area but as for the trousers that seemed undisturbed, tornadoes can behave in that way. Tornadoes can completely destroy things while leaving some things completely untouched.