As far as I can tell, the trend was started by Belgian Knee Warmers. Since then I've seen it a bunch of places: In Bicycling, on blogs like Competitive Cyclist, etc.
I have some thoughts on the use of this all-caps word, but first, as usual, I need to digress and share some context.
I lived in Boulder for about 10 years where I fortunately and unfortunately spent a lot of time with other cyclists. If you haven't been to Boulder before you need to understand that in the Gore-Tex Vortex, style trumps everything. The way you look, what you wear, what you're riding, where you eat, what you eat, your accent or lack thereof and more all contribute in some twisted way to Boulder's unique social stratification. In Boulder, wearing this year's Oakleys or Pearlizumi isn't cool enough. You need to get your hands on next year's stuff, even though it's unavailable.
Anyway, while in Boulder I observed many a Category 1 prima donna primping in front of the mirror for a good half hour. And no, they weren't prepping for a night out at the Rio, they were prepping for a group ride. They had to get their cycling cap angled just right. Or their trucker hat. Or wait for their new and extra-tall socks to finish drying, even though their closet drawers were stuffed with 50 other pair, but that were 5 mm shorter.
At the time--this was in the late 90's--it was all about being "euro." It was sad to watch the amount of energy guys would spend to try and look like what they thought was some ideal for a cyclist. Mostly they ended up looking like idiots with their sweaters tucked into their pants, listening to bad electronic music they pretended to like, while talking about how "super" racing was in Europe. (This is another euro thing. Super is an essential adjective/adverb. "He was super strong." "It was a super long climb." "That's super fantastic." "She was being super lame.")
Of course every now and then a bunch of said riders would actually make their way over to Belgium or France or Spain for a few months. And 9 out of 10 of them would come back totally demoralized, quit racing and trade in their bikes for a Vespa.
OK, I'll get off this tangent. My theory is that "euro" was the precursor to "PRO." And the irony is that every time I see a PRO reference it screams amateur. Or is it AMATEUR?
For example, I was reading a certain online retailer's popular blog recently. It referenced the very cool and PRO setup of running a compact crank with 51x36 chainrings. Really? This is PRO? Now I think compact cranks are a great idea, but when is the last time you saw a pro riding them?
Or head over to the weightweenies forums. It's filled with PRO references. But most of the things they allude to--like "that red SMP saddle looks totally PRO on your new Crumpton frame"--is pretty far from pro.
Anyway, hate to be a hater, but PRO is a pretty odd trend to me. Please, stop typing it.