Some species of dinos will grow and multiply faster when the pH is increased higher then 8.5, so if you elevated the pH above 8.5 this may have increased problems for your particular specie of dino. I would try the lights out with normal pH this time and see what happens.
Randy indicates this possibility in this article:
Problem Dinoflagellates and pH
http://reefkeeping.com/issues/2006-11/rhf/index.php#2
From it:
"It should be noted, however, that one species of dinoflagellate, the planktonic toxin producer Alexandrium catenella,2 was found to grow optimally at pH 8.5 in lab cultures. So raising pH is not a panacea for all dinoflagellate species that might be a problem."
FWIW, there is new scientific research that has been completed since Randy's article that has found that many more dinoflagellates tested will bloom at a pH level higher than 8.5.