Note to Commenters: Since BMX is nearly 50 years old, the question is not breaking the 20-year rule, as long as you keep your answers to historical developments and. You can address any development up to and including 1995 in your answers.

There's a motorcycling film called On Any Sunday (with Steve Mcqueen) that included scenes of kids riding BMX bikes on dirt tracks. IIRC these events were added to motocross races to allow kids to participate. The film is said to have spread BMX around the country.

Here's the trailer, BMX is the first scene, and you can see the types of bikes used at the time: https://www.youtube.com/watch?v=xSLRlPiiiQE