Why are the leaves on my viburnum falling off
Leaves are not maturing. They are small, turning brown, curling up and falling off.
There are many possible reasons for leaf drop in plants. This article will give you a general overview: https://www.gardeningknowhow.com/garden-how-to/info/plant-dropping-leaves.htm
For more specific information on viburnum, see this one: http://www.clemson.edu/extension/hgic/pests/plant_pests/shrubs/hgic2057.html
As you can see, there are many possible causes. The best thing to do is to take samples of the browning leaves as well as samples of green leaves, and soil samples, to the nearest Extension Service for analysis. This link will help you locate the closest one: https://www.gardeningknowhow.com/extension-search/