sorry for repost - Image reaper is right, because they are "mobile" nutrients the plant can move them from place to place, where and when they are needed, you will notice it's mainly on the fan leaf stalks (see my log) not usualy on the stalks that bare buds.
The plant is giving all it has to the buds. Lack of K (potassium) - it's okay unless it's realy bad.
Good luck - Hall