Post-exercise induced asthma is nothing new. You're right, exercise induced asthma is more common, but asthma after exercise does exist.
I've read things supporting low pCO2 levels (through increased respiratory rates) as a cause. When you breathe while exercising, your pCO2 level doesn't drop that much because your body breathes just enough to blow off excess CO2 produced from exercise. However, as you stop exercising and start to rest, your continued hyperpnea (breathing fast) causes your pCO2 to become low. This has been postulated to cause post-exercise asthma.
My personal belief (and I do emphasize personal because I do not have any evidence to support this) is that post-exercise induced asthma occurs when catecholamines return to normal. As you exercise, especially strenuously, your body produces catecholamines (epinephrine and norepinephrine), whose beta-agonism causes the bronchi to dilate. When you stop exercising, catecholamine levels return to normal relatively quickly (half-life of about 5 minutes). As the bronchi return to their normal size while you are continuing to breath fast, this causes "air trapping" in the lungs and creates the hyperinflation that occurs with asthma exacerbations. Therefore, you develop post-exercise asthma.
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