Why causes people to gain the weight back after fad diets?

No behavior change: After a fad diet, they go back to their old behaviors. It can be binge eating, eating late at night, etc. You don't learn lifelong techniques on making good and healthy choices with fad diets. Behavior modification with moderation, variety and balance are important to maintaining the weight loss from any program. Eat less, move more.

Diets: Fad diets are marketing ploys to promote a concept that has more to do with making money than with a well-balanced diet that promotes long-term compliance conducive with a healthy lifestyle. Many of these diets are also low calorie diets. Once the diet is completed and you return to your usual eating habits, weight returns.