Holding backspace brings up shut down dialog

Hi I'm having a strange problem on my macbook late 2011 13in. I believe there may have been some water damage to this mac in the past as the onboard keyboard suddenly started manfunctioning, and the keys became quite jumbled. I'm typing this on an external keyboard. This is what happens when I press G twice on the onboard keyboard: adsadfghs

Anyway, I was happy enough with the external keyboard until just today it has started to bring up the shut down dialog and go to sleep every single time I hold backspace for two seconds. It doesn't happen if I tap it a few times.

Anyone have any ideas what might be happening? Perhaps there's some way to disable this shortcut I'm inadvertently activating?

Thanks!