Horses, like people, have bad days. Horses feed off of the emotions of everyone around them, an overly sensitive horse will feed off a disgrunted stall cleaner, and just like the emotions of humans, horses can also feed off of the weather. My friend's horse used to get very dominant when the weather was bad (rainy, windy, or just very cloudy).
And as you mentioned, some horses just perfer one saddle to another. My little mare can be ridden english, if I put an english saddle on her and girth it up off she goes cow-hopping and bucking all over the place, but with my western saddle, I could pop a balloon on her back and she'd give me the "omg really?" look that teens give their parents after the parent does something embarrising to them.
Good for you to be willing to change for her. I used to be a straight dressage rider before I bought my oldest horse, and had to switch disciplines for him. It was hard, but now I love riding him.