I have had this kind of thing happen for two different cockatoos - very sad they are one of my favorite fish.
In my case, neither of them survived. (sorry)
They both developed it following a water change - because they are so inquisitive, i assumed that they may have gotten in the way of the gravel vac and had a bump - but I am pretty careful to keep away from them all...
I make sure my water going in is very similar in temp to the water allready in the tank, and in fact put it through the filter, so it is mixed thoroughly before it enters the tank at all. Also, I do water changes pretty frequently, so there won't have been any dramatic fluctuation in water parameters that could cause it.
I'm giving cockatoos a rest at the moment - too sad to lose them.