My friend's mare is a bit like that. She is extremely gentle and sweet most of the time, when she is in season she is the devil. At least to her owners..
My friend keeps warning me that the mare might bite, and that she turns her back quite often, etc. I arrive on the same day, and the mare hears my voice and appears. comes to me, doesn't try to bite, no threats to kick. I have been told it is because she respects me more, so there must be a couple of things the owners are doing wrong, to make the mare try to control them. She has made her owner move her hay to a place she prefers.. and of course I told them it was a big mistake. Soon enough we will get the mare under control again. Actually, I have been there when she is in season, and the worst was threatening to nip me when I was stroking her all over.. but then I reminded her that it is not allowed and she stood there. At some point she walked away - that's the thing, being free in the paddock, she will choose to walk away from me if she is grumpy. Her owner, well, the mare chooses to attack or just make them feel uncomfortable.
For me it has always been easy as I put the mare under control from day 1. I told her my rules and up to now she is respecting them.