[ChuckS]

I'm wondering if it could be sympathetic coupling. Specifically, the open b string (or fretted b note on the g string) setting the low E string in motion. That b note frequency is 3 times the frequency of the open E.

Try damping the low E string when you play the b note.

There is a similar relationship to other notes and other open strings. The high e string frequency is 3 times the frequency of the open A string. The a note on the 5th fret of the high e string is 3 times the frequency of the open D string. One of my guitars has a high level of this sympathetic coupling and I can hear the open E, A, or D string vibrating if the 'related other note' is played.