There might be wood-boring insects (likely a beetle, if anything) causing damage to the stems, but from the photos we can't determine this with certainty. Wood-boring insects often target trees/shrubs under stress (not vigorously-growing healthy plants), even if the plant doesn't appear to be manifesting signs of stress at the time the insects arrived. Drought is a typical trigger, but over-watering (or soil that drains poorly or stays too wet) is another. We also often find Viburnum branch dieback due to Botryosphaeria canker, a fungal pathogen opportunistically also attacking plants under stress. It's possible both issues will overlap on the same plant, though it can be hard to well which form of damage came first in that case.

The only recourse for either situation is to trim out affected branches, as you have done. Shrubs with roots that are otherwise healthy and vigorous should be able to regrow new branches, though those might take a few years to mature enough to resume flowering. Monitor plants for watering needs when we're in stretches of hot, dry weather (this spring was becoming dry of late, until recent rains) by feeling the soil around six inches deep in the root zone. If somewhat dry to the touch at that depth, water well if rain is not forecast. If mulch is used under the plant, make sure it's not piled up against the main trunk/stems, since the bark should get good airflow.

Miri