What's going on with my cat running back and forth crazily?

Possible reasons:

Practicing hunting: Cats are naturally hunters, and they may run back and forth to practice their hunting skills. When they see a potential prey, such as small insects, mice, or lizards, they will chase after it. If their owner doesn't notice, they might think the cat is running crazily. To confirm this reason, the owner can look around or on the cat for any signs of prey.
Parasites: If the cat has fleas or other external parasites, they may feel itchy and uncomfortable. To alleviate the discomfort, the cat might choose to run back and forth to shake off the parasites. The owner can carefully inspect the cat's body for signs of parasites or their feces, and deal with them promptly if found.
Burning off energy: Cats have a lot of energy, and if they spend most of their time sleeping, they might act crazy around the house after their owner comes back to burn off excess energy. In this case, the owner can spend more time playing with the cat to help it expend its energy, and the cat will naturally rest when it gets tired.
Getting rid of smells: If the cat runs crazily around the house after defecating, it might be because they are worried that the smell of their feces will reveal their location and attract predators. Therefore, they try to shake off the smell by running around. This is a normal phenomenon, and owners don't need to worry too much.

Remember to always consult a veterinarian if you have any concerns about your cat's behavior or health.