At least a big part of what's going on isn't actually Nikon's doing, but Sony's sensor division (which benefits Nikon, Fuji, etc. as well as Sony's own camera division). Sony is well ahead of anybody else in the manufacture of large CMOS image sensors (anything bigger than a phone sensor is "large"), and they sell those sensors to whomever asks. Those asking include essentially the entire camera market not named "Canon" (there are weird exceptions like some Leicas).
Sony-sensor cameras have a large dynamic range/noise advantage over Canon-sensor (and the rare other options) cameras. Within Sony sensors, the choice between Sony themselves, Nikon, Fuji and the smaller players comes down to lenses, familiarity and compatibility with what you already have. Given similar sensor size and pixel count, image quality is going to be very similar with a similar lens. Sometimes, even sensor size doesn't matter that much - Fuji, with their beautiful APS-C lenses, has come surprisingly close to the image quality of 24 MP full-frame cameras.
Canon's obvious choice is to keep making great bodies and lenses, but buy their sensors from Sony... So far, they've been too proud to do it - but there are consistent rumors that they might.