Wrong app is focused when quitting another one?
Just asked the same question and saw yours after…
It's actualy not related to Finder/PDF/Preview:

I had the issue with a first Mac (iMac): every time an app was quit (auto or by me), MAMP software (that I always let hidden) was reactivated. I thought it was a MAMP bug, but I just updated to Mavericks on another Mac (MBP – yes, I'm a bit late), and exactly the same thing is happening with Firefox (I have MAMP installed on it too).

So I just think this is a Mavericks bug…
https://developer.apple.com/bug-reporting/ !