What are the reasons that people follow so called "fad diets" like gluten free even when they don't have celiac?

No reason: There is no reason to be on gluten free diet if you do not have celiac disease.

Consumer trends: Consumer trends have long held popularity. Gluten free is the latest. People like to do what's cool and hip. I would say, however, that if you are picking a food group to skip, then bread would be the smartest one to do without. Gluten free living, so long as you don't substitute it with tons of gluten free cookies and baked goods is a good way to go with lean meat, veggies, fruit, milk.