Do you remember having certain bikes in fashion at certain times? To me it looks like bikes have now fallen to the same pit fall of fashion fluxes as everything else. You know, fashion is a way to keep the market economy growing: you have to buy new stuff all the time when the old goes out, even if it's still usable. Although, if you are smart, you can trick the system – it goes in circles! Those bikes that were in fashion decades ago are the hottest thing right now :) So ask your mom or uncle if they have “vintage” bikes, and you'll have the hottest bike in the neighborhood.
Was there a time before bike trends? I never noticed them until the 2000's. I thought that a bike is a bike is a bike. When it's got 21 gears, it's a fancy bike. However, now judging by the variety of so many different retro bikes, other people must have thought differently. Anyways, the first trend that I remember started from a music video. I don't remember what the band was or when it was shot. It must have been in the beginning of the last decade. It was a retro music video. A bunch of boys were riding their cruiser bikes on a hot summer road. The next summer cruisers were everywhere, and apparently they did come to stay, because they are still everywhere.