Hi Peter, thanks for the question.

This is not unusual for mulberries or fruit trees in general. Mulberries can drop fruit prematurely for several reasons. Wind, heavy rain and extreme temperatures can cause this type of drop. These trees will drop fruit as a natural thinning process so the tree can focus resources on the remaining fruit. Fruit can drop due to pore pollination as it can for a tree stressed by disease, water stress, or nutrient issues. Young mulberry trees will drop fruit while it is getting established.  

I hope this is helpful.