Probably due to not keeping it evenly watered but looks fine even with yellow leaves.

Heat stress in the summer could make some leaves turn yellow. Lack of water would show up as leaf edges turning brown and growing brown inwards. Some of the leaves that are green appear to have leaf spot as a few of the yellow ones. Hard to say which came first as leaves that are yellowing and browning sometimes show these leaf spots more obviously than dark green ones do.

The leaves do not look like a nice dark green color. But that may have been the camera's fault. They were also too far away to tell if you have iron chlorosis so, check the soil pH to see if it is becoming alkaline or if the soil has a nitrogen problem. There are kits sold at plant nurseries that will check these things for you.

Try to maintain the soil as evenly moist as you can with few or no periods of dry/moist/dry/etc. Try not going overboard with moisture (that is, wet soil) as too much moisture feeds the spores. Dispose of the fallen plant debris in the trash. Water the soil (in the early morning hours) and never the leaves.

That shrub is at an age where you do NOT have to manually water it, unless in a severe drought.

I have many paniculatas and the one I watered the least has no yellow leaves.

Thanks very much everyone. Luis, I agree they are not a "nice dark green," rather a medium-light green. I fertilized everything in the spring with MG, then left them (lazy gardener!). I will attempt to check the soil pH. Google informed me as well that I may have overwatered.