When a star enters the red giant stage what causes its outer layers to expand?

Well, honey, when a star enters the red giant stage, it's all about that helium fusion party in the core. As the star runs out of hydrogen fuel, the core contracts and heats up, causing the outer layers to expand like a balloon on a hot summer day. So, basically, it's the star's way of saying, "I'm getting older and bigger, deal with it!"