They seem to have plenty of leaves on the outside so presumably it's just the normal aging and dying of the oldest leaves - broadleaf evergreens often shed these during the growing season rather than in fall or fall only. Sometimes for months at a time, as with Magnolia grandiflora.

When it looks like there is a problem with old leaves dying off is when shrubs are starting to become see through (gaunt or sparse looking) because of it. A common cause of this is soil moisture problems, either too much or not enough. Otherwise too many leaves aging out and dropping at the normal time, when there is not a soil moisture problem discovered can be a soil mineral deficiency response. Lack of nitrogen for instance, with the shrub concentrating what minerals it still has present within it in the youngest leaves nearer the shoot tips.

@Embothrium - thanks! I'd be relieved if it is a normal process that we're just not used to. I will keep an eye out for them thinning.

None of the them appear to be thin. ...Not with all the growth put on from last fall to March - shockingly so. (may be attributed to Hollytone plus mild winter).