Well, I guess there's two kinds really. There is a more formal version such as the Squadron does. The Squadron is a group of Brothers that assembles themselves for the express purpose of bombing an unsuspecting target once a month with tobacco, pipe related stuff and sometimes some other random selections. They assemble through the Community in Action section. You can see their work there.
The more informal bombing is when a Brother simply chooses to bomb another Brother at random. Sometimes anonymously. You can find threads in the Community in Action section where Brothers will explain how it happened to them.