What is a passing trend?

A passing trend is something that used to be insanely popular for a while, and has kind of faded, or "passed" away. Its not nearly as popular. For example, backward caps were majorly popular in the late 90's and early 2000s, but now, almost no one wears their Baseball cap backwards. Its faded and almost disappeared. But I'm sure that it will resurface again, maybe not for a while, but it most likely will, because almost all trends do. Its kind of like a cycle.