A lone female American Wigeon hanging out with a group of Blue-Winged and Green-winged Teals.
This bird stayed mostly with the pack of Teals and occasionally nipped at them, which didn't seem like the friendly thing to do,
Later I read that Wigeon often gather with feeding coots and divers and are notorious for grabbing pieces of vegetation brought to the surface by diving water birds. For this reason, they are sometimes called “poacher” or “robber” ducks.
So this is what the nipping was probably about.