This shrub is usually tough as nails so I understand your perplexity at them not behaving uniformly.

I'm not sure there is any trick involved but the degree of pruning and timing may have some bearing. On a 'mature' shrub with wood 10 to 15 years old, I'm not sure I'd cut them to the ground but to manageable low framework, say somewhere around 12-16", maybe lower (hard to be precise without seeing the plants). Generally, the cutting back to the ground practice (coppicing) should be started before the plants are that old. The cutting back to a low framework is a pruning technique known as stooling.

And I'd wait for early spring to do this.

There is an alternative method of going in and removing just the oldest growth at the base, typically removing no more than a third of the plant at a time. This will encourage the production of new basal shoots.

Do your plants produce suckering shoots? Cornus sericea is known for this type of suckering, spreading growth habit.

Thanks for your response. Yes I believe they all or mostly all have suckering shoots at times. I gather the previous owner just sheared them from the tops (10' approx) for tall privacy hedges. I'd like to have them as lower shrubs for the red stems, so once they vote back will continually prune out older/grey stems.
Spring is ideal!