Right after eating and drinking, with head shaking, in birds that are otherwise loud and busy is the normal version. Their nostrils are on top of the bill and they dunk specifically to rinse feed and dust out of them. The sneeze and the head shake are the rinse finishing.
Crumbly starter feed gets up there, so does dry bedding, so the two moments you noticed are exactly the two you would expect. Five out of five doing it is not a cluster, it is five ducklings with the same bill.