This is pretty normal for a plant in mid flower and is not something to be overly concerned about.
What is occurring is that the plant re-uses carbs, sugars, amino acids etc that are in the older leaves to be used as a source of nutrition for growing the flowers. It does this because it is far more efficient to recycle these elements, or "re-purpose" them, than it is to grow flowers AND make new carbs, sugars, amino acids etc from new at the same time as growing the flowers.
Had you defoliated your plant, these vital growth elements would not be available for re-purposing, so the plant would have to divide its' energy between growing the flowers and making the growth elements from new, so congratulations to you for letting your plant be as natural as possible!
Congratulations too for the clear and concise question, to the point and without the waffle and piffle. How refreshing!
Hope this helps, Organoman.