Helo Jordan.

There are many causes of leaf curl with viburnums.  This can occur due to aphids, spider mites, thrips, a type of caterpillar or even beetles.  It can also be caused by too much water, poor drainage, too much direct sunlight or type of fungus known as verticillium wilt.  

V wilt is a fungus that lives in the ground and lies dormant,  Once the fungus germinates and infects the roots of the plants through wounds or natural openings. "The fungus spreads into the branches through the plant’s vascular system and simultaneously causes the plant cells to “plug” themselves. Once the xylem is infected, it becomes so plugged that water can no longer reach the leaves.Verticillium can also be spread to plants through wounds on branches or trunks."  It is very difficult to control, and can be spread from plant to plant via infected tools, so sanitizing tools is very important. 

Because the entire plant seems to be affected all at once, I don't believe this fungus is the cause of the wilting.  However, if you do witness any branches becoming brown or dead, be sure to prune them out right away.

More than likely, this is caused by aphids or some other type of insect.  Sometimes the damage is done and you don't even see the actual insects feeding.  Try unfurling one of the leaves and inspect it carefully.  This problem is easy to treat and using insecticidal soap should do the trick. You may need to apply several applications. 

Keep me posted! And thank for your question