Sympathetic Resonance is the issue. I have tuned guitars in the studio and then played forceful passages in the same room. What you hear after abruptly stopping is the stings and body of the guitars, violins and snare drums ringing due to the acoustic waves that are being produced and flowing/reflecting throughout the room. Similar to a slap echo you may produce by aggressively clapping your hands as you walk around and excite frequencies in the immediate space.
