Cropdusting is a form of air pollution.
Specifically, it is the act of farting while walking through a crowd of strangers. One may excuse oneself from social gathering specifically for the purpose of ambulating about until the gas has cleared from the anus. Doing so makes identifying the culprit not so easy for those subjected to said cropdusting. The cropduster's friends are thus spared a possibly unpleasant odor while the cropduster saves face.
By now you have certainly ascertained that cropdusting is the sort of neologism to be celebrated by afficianados of Urban Dictionary. But do know that the first recorded usage (while still anecdotal) was by none other than David Sedaris (in his piece Standing By) who was informed of the practice by a snarky flight attendant whilst aboard an airplaine. It goes without saying that the presence of a confined airspace makes cropdusting all the more offensive.
At the risk of falling off of my high horse like a hypocrite, I denounce cropdusting as a terrible thing. Strangers do not deserve to smell the farts of others.
Why should one feel the need to hide a fart from a friend?