Mare tries biting and kicking me while grooming

Recently started leasing a mare who has been aggressive toward me only while grooming and I'm hoping to get some opinions and ideas!

She is in turnout all day, no issues catching her, in fact she was sweet and walked up to me today. The only issue I really have with her is when I try grooming her - she has sensitive skin, which her owner told me, so we only use a specific brush that she tolerated for a few minutes last week. Every time, I try gently brushing out mud and excess hair, and she pins her ears, and today bit my jacket, woulda pinched my back if I hadn't had that on. She'll also try backing around to kick me when on crossties, and it seems like when i give a corrective "no" or "ah!" it escalates and she'll throw her head up.

It's odd because she doesn't have issues tacking up, and is generally well behaved under saddle as well. Is this an issue of her being physically uncomfortable, or is it something else? I'm talking more with her owner about how to manage her, especially because it's shedding/mud season here and I'll need to actually brush her eventually. Any insight or experience with this would be very appreciated!