To a man with a hammer, every problem looks like a nail.
Well this is how it applies to leadership. If you have just one tool (i.e. leadership style) in your tool kit, you'll default to using this even when the situation demands a different approach and the results will be sub-optimal.
It's kind of like trying to saw a plank of wood with a kitchen knife, or put up a shelf with a spanner. You get the picture.