They could be eating too quickly too. My birds sneeze if they eat too fast because they're trying to dislodge food particle. Just something to consider.
It's very possible that they have allergies too. Is their brooder dusty? Clear, nonsticky fluid is okay every once and a while. The head dunking could explain it all too. Silly chickens. Watch them, if you see any thick, off-colored discharge than you should worry.