I believe it is a cause of stress, myself. I had to cut a budded plant early one year, and hung upside down for 2 or 3 days. Then, I Cut off the individual buds from the stick and hung them upside down, and most of the hairs on the undeveloped bud went from a milky white to a strong red, in one night. So, my guess is a stressing, cutting kind of thing..