If you use headphones with a TRS connector, you might also have noticed that if you plug it into your music player and pull it out halfway, the music sounds strange. It seems as if there are some frequencies missing. Why is that?
I once had a possibly similar experience when I forgot to connect the speakers' ground in an amplifier. You then get just the difference between the two stereo channels. If you extract the connector ground is probably the first to be cut. Depending on the mixing some parts of the signal (some frequencies) may be placed in the center of the stereo image, and those will be filtered out.