Apparently my gut instinct was right and Nina's appalling and sudden lack of ground manners was a bid for dominance. She has become quite agreeable about riding but had to make a last ditch effort to be the one in charge. It is, after all, her norm.
How can I tell? She is pissed at me. She pins her ears and stomps her feet and glares thru her one good eye (the other is still a little swollen) and makes mildly threatening gestures. Then she comes over to me and wants to be friends and get her head scratched. She has been pushing people around for so long she doesn't know how to give it up. But I think the secret is that she really wants to and she will, on her own, eventually.