A lot of people seem to be having this problem on all forums. I'm thinking with the climate changing like it has that the increased warmth and humidity of most places, with a lot of rain in others, is causing more mold spores than used to be.
All of our trees are growing fungus/lichen the past few years and many of them dying and I think it's much to do with this greenhouse affect on the Earth.
My Orpingtons sneeze when they dunk their head into something powdered (usually the little bit of spent grower feed my chicks have refused to eat that I then give to my food-crazed hens as a treat.) This can last a few minutes. It just proves they're... not the sharpest spoons in the drawer. This might be something to consider if they're only sneezing at feeding time, particularly if you feed crumbles.