I do not have even remotely close to the knowledge that others have on here, but I noticed something with my poor solitary gosling that might apply. I had three geese on nests, for the most part they don't hatch well. That one gosling hatched from the one nest, another looked pretty dire but one looks promising. That's sort of beside the point but I'm noticing that my non-hatched-yet ganders from the pairing are starting to get feisty with each other, and just about everything else. I think the non-fathers want babies.
EDIT: Oops early send
And since that gosling only has his generally angry parents to learn from, I actually caught that little lemon trying to snakeneck a full-sized scovy duck. More than once.
Maybe with yours, the adults are nippy and they're learning behaviour from that?
But again, so far from an expert...