She probably just needs that extra bit of ground work prior to a ride, especially if she doesn't get worked often enough. Some horses are just a little more 'frisky' when they've had time off (my mare is), and so it is a good thing to make sure they are listening to you before you hop on.
Hahaha...I agree with Velvet, in that a super consistent horse would be boring...Lol! NOT that I like bucking, rearing or any of that nonsense (though sometimes it just happens), but I like to know my horse is still "alive" and still can act like a horse from time to time; plus it gives me an idea of where I may need to tweek things, as when a horse is "perfect" all the time, you may not see bad behavior coming until it slaps you in the face, or lands you in the dirt.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."