A thing, idea, or whatever that is done/used only because other people do it. Douche fad is used as a negative term, because it implies that people that are part of a douche fad cannot think for themselves or be themselves.
If you see a lot of people doing one thing that really annoys you, and can see that if only one person did it it would make that person look like a total idiot, it is probably a douche fad.
Understand that not all fads are douche fads. "That's what she said" is not because if used properly, it is genuinely funny. Other fads are cool too.
Examples of douche fads include:
Wearing your pants so low your boxers/thong is showing
Using overly used words like "awkward" (<--especially when it is misused) and "creeper". "Ignorant" also fits into this category
Doing the cool walk