harmonics
Yes, it does have to do with harmonics. I'm not an expert on this subject, but we are all, I guess, familiar with the story of the opera singer who could break a glass by singing the correct note; in fact, there was a commercial on TV some time ago which purported to show that. From my own experience, I noted an interesting manifestation of this: last year, on a big festival, our organist at church, as postlude, treated us to the Toccata from Widor's 5th Organ Symphony, a well-known piece which reminds me of a bus blowing its horn while going through the tunnel under the Simplon Pass (I've heard that, so I can make the comparison) and ends up on the tonic major chord with all stops on and everything wide open. As he was playing this, I noticed that the big brass altar cross was vibrating with the music. Not having ever played that piece or seen the score, I had to ask what key it was in to see what that piece of brasswork was tuned to.
