>IMO, it's because Canon is finally shipping the 6D.
Interesting comment, because this presumes the buyer would be choosing between a 6D and a D600. Buyers who already have Canon or Nikon lenses would, presumably, not be making that choice because they're already into one system or another.
But, the really interesting part of it (at least, from my perspective) is that if the users in either case have a DSLR with lenses designed for APS-C (DX, in Nikon-speak), then the choice is reasonable. Thinking about my own case, most of my lenses are DX-specific, so if I were considering a D600 that would mean getting rid of most of my current lenses and buying new ones. And, if I were doing that, it would seem reasonable that I'd look at Canon's offerings along with Nikon's.
Canon is the same as Nikon, of course; they have APS-C-specific lenses too. So, if this is a factor (seems reasonable), it must be because both Canon and Nikon see these cameras as ways to get the users of one brand to switch to the other.