If it was cool and wet during bloom it is difficult for the bees to pollinate the flowers. Since Rainier cherry needs another variety to supply pollen, the bees may not have done their job. Another possible problem, although less likely, is the cool wet weather allowed a fungus to hit the flowers and cause them to turn brown and fall. I am assuming that you do have another cherry tree near to allow the cross pollination. Bing, Royal Anne, and Stella provide good pollen for Rainier.