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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/

The only reason would be that you were sold a plant that was mislabled. It happens. A careless greenhouse worker can add the wrong tag to a batch of plants and they are sold as the wrong variety.

If it's just going to be a mild frost, I would recommend covering the plants with a bedsheet. This should be enough to keep the frost from settling on the buds. If you can, a few jugs of hot water placed under the sheets as well will also keep the air a bit warmer under there. If you have not read it, this article will also be helpful: https://www.gardeningknowhow.com/plant-problems/environmental/frost-how-to-protect-your-plants.htm For freezing tips, this article will help: https://www.gardeningknowhow.com/plant-problems/environmental/protect-plants-in-freeze.htm

You may just need to prune off any dead growth. This article will help: https://www.gardeningknowhow.com/ornamental/shrubs/viburnum/viburnum-pruning.htm

The browning could be from unsual temperature changes or a fungus. This could also be inhibiting the blooms. Typically, they need lots of sun for blooming too, but you can try adding some bone meal to the soil around your shrubs to help encourage flowering. Also, cut out the dead, brown foliage and treat it with a fungicide, like neem oil.

Your soil may be lacking phosphorus, which aids in blooming. You can take a soil sample to know for certain, but it will not hurt to add some phosphorus rich fertilizer or bone meal to the soil to encourage blooming next season.