Sorry for the confusion earlier. As I mentioned I have come across the odd string of jpgs and this happened to me certainly by the process I suggested. Set to FX, the camera does produce an image covering the whole viewfinder but at a reduced pixel weight - one that could be confused with a DX size in post. But you report having Auto DX Crop set on, but with no memory for having done this. No mystery here, that is Nikon's default setting (default setting are listed on p 312-3). However, along with that setting you would have also had to assign either the preview or the fn button to Choose Image Area. In fact, Nikon had already done this for you - that is the default setting for the fn button. Again, no mystery. So the sequence you must have followed is pressing the fn button (7 o'clock around the lens seen from front) and rotating the main command dial (thumb dial). The trick now would seem to be to learn not to do that.