I think the bike boom was part of a larger "outdoor" phenomenon that happened in the 70's. Camping and hiking were immensely popular as well. Every college and high school student was wearing hiking boots (a.k.a. "waffle stompers) as well. Totally impractical as everyday footwear - very hot, heavy, and heaven help the poor soul who stepped in a pile of dog droppings. Yet we wore them, in spite of my Mom insisting that I "take those damn things off" before I came in the house due to the dirt chunks they left everywhere. You can also find millions of 70's style external-frame backpacks at garage sales for next to nothing (sound familiar?)
Fads come and go with no obvious reason. A far more ridiculous fad was the "cowboy" boom of the early 80's inspired by the "Urban Cowboy" movie - everyone (even in mid-town Manhattan) wearing cowboy boots and hats, and every bar had to have a mechanical bull.