I've never seen her bite or nip, so this seemed odd. That said, I have seen two potential signs though that didn't make me 100% surprised (just 98% surprised...).
1. She's been making nasty faces, even at me, when the blanket goes up over her back. She's always been girthy and these are the same "don't do that" faces she makes during saddling (back and saddle fit checked recently, all's well).
2. When she's in the aisle on crossties and he walks by she pins her ears and gives him her nastiest mare face. This has been in the past week- so something's going on between them.
He has a lifetime of horse experience and I trust his horse handling skills. He said he did get after her when she bit, but I absolutely do NOT want this escalating. I wasn't there to see what happened in the moment, so I'm not quite sure if I would have done anything differently (though again, I trust his instincts). So...any advice on staying on top of this and not let it get worse?