I wouldn't read too much into that trend, all desktop operating systems declined:

http://www.google.com/trends/explore?q=Mac+OS+X,+Linux,+Wind...

Ubuntu really does win though among the linuxes:

http://www.google.com/trends/explore?q=Ubuntu,OpenSuse,Debia...

My favorite linux is on the rise, albeit from a tiny baseline:

http://www.google.com/trends/explore?q=Archlinux&cmpt=q

At least the mobile OSes are doing well:

http://www.google.com/trends/explore?q=Mac+OS+X,+Linux,+Wind...