Nikon losing the leadership position to Canon in the market for professional photography is a story involving technology transitions over a period of 30 or more years as told in this annotated twitter thread. medium 
Nikon, originally Nippon Kogaku, Japan Optical, started designing lenses for the leading rangefinder cameras of the time, Leica.
Nikon took a very conservative approach to adding features having secured (and defended) the professional market. 8 years later they released the Nikon F3 which *required* batteries for the *first time*, but even had a backup mechanical shutter release that was heavily marketed. As crazy as it sounds, professionals, especially National Geographic types on assignment, were terrified of running out of batteries so Nikon catered to them.
Canon took a bold step and designed a whole new lens system around *autofocus*. It was vastly criticized by professionals and the press at the time for breaking compatibility with Canon’s existing FD mount (which itself had maintained compatibility over 3 generations of Canon lenses going back to the 1960s). This lens mount made driving autofocus much more efficient and fast and also easily supported automatic exposure known as “shutter priority”.
Seemingly overnight Canon captured the professional market. While numerically they have about 60% market share, they have a much higher mindshare. Disruption happens two ways—first slowly and then quickly.