The saying, “it was so quiet that you could have heard a pin drop”. Why would a pin be dropping anyway?

Gravity

Shouldn't we be asking what the pin was dropping?

It was so quiet that you could have heard a pin drop an angel from its head.
It was so quiet that you could have heard a pin drop names.  
It was so quiet that you could have heard a pin drop a hairpiece.
It was so quiet that you could have heard a pin drop a diaper.
It was so quiet that you could have heard a pin drop a hem.

To see if you are listening or to test your hearing. 

When I was in school, pens were constantly dropping.
People dropped pens in the halls between classes, pens would drop off desks, sometimes pens would even drop from the ceiling.

I like Shuhak's modifications! :-)

It presumably refers to a haberdashery pin and from the days when it was routine to repair or even make, one's clothes.

You'd have to have very keen hearing and no  background noise to hear one of those drop even on to a hard floor!