I've heard some sarcastic remarks about that as well -- "How do so many peak fitness athletes suddenly develop 'asthma' and need inhalers?"

There are a lot of valid reasons. They're exposed to a helluva lot more airborne contaminants kicked up from roadside dust than most folks. The most common aggravators include mold/mildew/fungus spores, along with pollen. I've seen otherwise healthy and fit asymptomatic people suddenly develop constricted airways and need ER treatment when they traveled to an unfamiliar location for camping, hiking or other activities. They hadn't adapted to the local airborne/dustborne contaminants.

Bike racers much contend with new locales and contaminants they've never adapted to all the time. I'd bet mountain bikers have a very high incidence of exertion induced asthma too.

And that doesn't even get into the possible contaminants from manmade stuff -- pollution, dumping, the usual stuff that comes off cars from routine driving. There's an incredible amount of tire dust along every road in the world. No idea how much that may affect some folks who otherwise never experience asthma symptoms until they're huffing and puffing in a peloton with other bikes, support vehicles and crowds stirring up the contaminants.