*I* had always thought that Miss Gulch, who transformed into a Witch
was the Wicked Witch of the West when she was up inside the tornado with Dorothy.
It turns out that Miss Gulch was in reality the Wicked Witch of the East and used the Ruby Slippers to transport herself from OZ to the Real World and back and used the tornado as a cover. What she hadn't counted upon was the Gale House being lifted up inside the tornado after taking a direct hit from it.
Miss Gulch owned "half the County" and was hungry for owning all of it.
One of the only obstacles which stood in the way was the Gale Farm which she wanted and even hinted at in the movie when she said, "I'll bring a darn suit that'll take your whole farm."
The Gales would have none of it. Thus, Miss Gulch decided that she needed help from her sister in Oz, the Wicked Witch of the West. That was why she was headed back there and that was why she was there when the Gale house suddenly and violently entered the equation.