The cutting of the grass causes the pollens to be disrtibuted into the air, Thus getting up our noses more easily and causing hayfever. (Like shaking a well used duster in the air)
I don`t know if there is any evidence to back up this theory but I have heard that hay fever has become more common since hover mowers and strimmers were introduced. This is supposedley because each blade of grass is now pulverised by multiple cuts instead of the previous single cut by a hand operated mower or a scythe, and thereby releasing more pollen into the air.
Only a theory.....but who knows?????