One cause can be very simple. Plucking and waxing can dry the tender skin around the eyes, including your eyebrows. A swipe of Vaseline at night can stop what seems to be "dandruff of the eyebrows."
Another common cause can be a yeast infection. Yes, that's right, a yeast infection. Not the same as the one "down below", but irritated skin can develop a yeast infection and it can persist if not treated.
Luckily, there is a very simple fix to this problem as well. At any discount store, purchase a tube of Lotramin cream. Apply it to your eyebrows and surrounding skin daily for a week. Be sure to avoid your eyelids.
