iTunes and Spotify push switching

Recently I was on firefox, and also had spotify and iTunes open, I believe I was playing music through spotify when i noticed that the music kept on switching to playing a song in iTunes, and then switching back to spotify.

My computer is an aluminum macbook, running 10.5.6. I have it up on a
Griffin Elevator Notebook Stand, and have an apple external keyboard connected to it, as well as some JBL speakers. I'm not sure what piece of that is relevant, but better safe than sorry.
I often ignore the secondary keyboard and simply use the laptop keyboard, even though it's on the stand.
I managed to figure out that the music was switching whenever I accidentally pushed down on the computer whilst typing.

So I started testing it out, and found that when I pushed down with both hands, lightly, without touching any keys or the trackpad, on either side of the trackpad, until the computer rocked forwards slightly, the music playing in iTunes would pause and the track open in Spotify would start to play, or if it was the Spotify track that was playing, when I pushed down on the computer on either side of the track pad it would stop and the Itunes track would start to play. In this way I could swtich back and forth between Spotify and Itunes, simply by pressing down on the computer, and that would occur whatever program was in view in front of me, whether that was Spotify, ITunes or a third program such as Firefox.
Does anyone have any idea how this is possible? I cannot find any information about it. Is it an accelerometer feature or something?
I took the computer down off the stand and it still seemed to happen, but after I unplugged everything it stopped happening, and I haven't been able to make it happen again.

While I can't say the usability of the feature seems that great, it was amazingly cool when it was happening!