Ask HN: Why does pressing the 'cmd' key expand SO's search bar?

I was just using StackOverflow, and as I went to switch tabs with 'cmd'+'TAB', the search bar expanded. I whittled it down to just having to press 'cmd'.

Does anyone have an explanation of why this happens?

NOTE: you have to have clicked into the search bar before pressing 'cmd', i.e. it has to be active.

see: http://www.StackOverflow.com