I feel like any "diet" that is named is instantly called a fad, and that really bothers me. Paleo is more or less eating whole, unprocessed foods, with an emphasis on low sugar intake. What's so faddish about that?
It's popular right now because some dude wrote a book about it and goes around doing speaking engagements about it. In a year or two, the people who are doing it now will move on to whatever the new diet is. That's pretty much every single "fad diet" ever.