I don't feel it is a fad right now, but that people have gone undiagnosed for so long. Years ago it was thought of as a child's "wasting disease", but now we are coming to understand that it is very prevalent but undiagnosed, among adults also.
A good book to read on the subject if you are interested is "Celiac Disease: A Hidden Epidemic". The book is by Dr. Peter Green. He is an expert on the disease.
The increase in available items for celiacs has less to do with fad and more to do with demand and an increase in DX of celiac disease.
I have a family member who has struggled for years with the gluten-free diet, but now with the increase in items, her life is much easier. Her DX did not come until she was 40 years old, and it did not present itself with GI symptoms. It was the more "silent" version of the disease. She had early onset osteoporsis (which is a symptom), and that is when she found out she had the disease. It does not always present with GI symptoms. I have become more informed by watching her struggle, not to mention having her to dinner.
Just know that, it is more prevalent than you think, and testing does not always show that you have it.