Maybe you were kind of sick already when doing the leaf duty and got enough worse to allow enough problem spots in your lungs or bronchia to consolidate. As far as I understand it (not much) getting all the way up to pneumonia is sort of like nucleating crystals or raindrops: if nucleation exceeds dissipation, the stuff being nucleated wins. Clearly, you haven't been being dissipated enough.

Best wishes for a speedy recovery!