Are their some things in the air (like different types of dust and molds) that can cause chickens to sneeze?
I think that things can make them sneeze just like us. I heard some people here say that pine shaving dust can make them sneeze. I don't know about seasonal allergies etc.
Yes! Too much ammonia in the coop will make them sneeze. If you can smell ammonia when you open the door, it's past time to clean it out.