Well best I can tell, they drop an M&M because it's a Mars factory and they're just known for M&Ms. They may not make them there now, but they probably made them there at some point.
A quick google search revealed some fascinating traditions:
The object-dropping craze is epidemic throughout Central Pa. Harrisburg, of course, drops a strawberry; Lancaster drops a red rose while York drops a white rose; Lebanon drops one of those god-awful bolognas; Hummelstown drops a sucker; Dillsburg drops a pickle (obviously); And Falmouth, home of the infamous goat races, drops a stuffed goat. Cleona, for some reason, drops a pretzel, Mechanicsburg a wrench, and Shippensburg an anchor. Manheim, instead of dropping a ball a la NYC, raises one.
Annville doesn't seem to be in the mix - maybe they could drop the poor departed Dutchman head?