Hmmmm, lets go into the facts and (not so much)numbers :)

1. There is water on the moon, in the form of ice but it is there.
2. There is no wind.
3. Moon gets hit by the meteors all the time, there is also a probability that a small meteor hit exactly on the astronaut's footprint. However small the possibility it still is, and can/will happen once in the future. :)
   Cheers ;)