Question

Why did Miss Kellner run indoors when she saw Uncle Ken at the wheel of the car?

Answer 1

The passage indicates that Uncle Ken has a reputation for being a troublemaker. It's indicated in the line by Granny as well, when she asked him if the hotel shut down because of him.
Also, his driving is described as reckless, as mentioned in this line: "Driving through the gate, he nearly ran over a cat."
He drove straight and fast, which means that there was a possibility that he would run over anybody that came in his way.
Hence, when Miss Kellner went outside and saw Uncle Ken driving the car, she ran back inside out of sheer fright.