Granny absolutely needs something to get furious about, and her broomstick fills this role. If she ever got a brand new broomstick (not that this would ever happen- she's a "second handy" person, not comfortable with anything new) within a month or so it would be behaving the same way, and she'd be swearing at the dwarves who informed her "it didn't act like that with anyone else". This, for once, is not the sheer bloodymindedness of the inanimate, but the thouroughly cowed inanimate behaving as expected- Granny just
knows the things going to play up, so it does.
