As has been said, the NCX shakes its head a bit if you let go of the bars at about that speed. Some owners report little to no shaking, some quite a bit. I get more shake if I have more weight on the rear of the bike (like a loaded top case), or if I eliminate static sag up front or have too much out back. I also have experienced varying amounts of headshake depending upon exactly what tire I have on the front. Not just what tire ‘model,’ but what individual tire. There was a combo of: tires + loading + sag settings I had a few years ago that made the bars shake so badly at that speed that they would _almost_ bounce off the steering stops.
All of this is to say that I believe you’re experiencing what you say you’re experiencing, that it wasn’t there before (or was very, very mild?), and that the tire change had something to do with it. What I’m ALSO saying is that none of those facts make this unusual for an NCX. They shake their heads some at a certain speed.