I would suggest that, although one could suggest that long-distance backpacking of long trails may be a fad, the use of trail runners as the most popular footwear is simply common sense gleaned for a more educated community with more experienced in long-distance backpacking. And sure, I've seen people climbing significant non-technical mountain peaks in minimalist footwear, and I would suggest that may be connected to the ultra-light fad we are living in. But, those same shoes used as long-distance trail footwear is more functionality than fad.