Appears to be deer browse. If so, nothing to do but replace with something less favored by deer.

Really???? I would have never guessed that in a million years. We do have deer around but never considered them to eat these. I'm shocked.

Arborvitae are sometimes called ’deer candy.’ Your plants have classic presentation - see how much nicer they look at the top, where the deer can’t reach?

Amazing... I did some googling for images and it's a match. You're amazing for calling that out. I assumed some kind of root rot or soil issue. Wow. A whole new issue I wasn't expecting. Thank you so much for pinpointing this.

Lots of nice, knowledgable people around these forums, Karl. It’s a good place to ask questions and learn!

You don't always see the deer because by nature they are a nocturnal animal and do most of their browsing after dark.

Thank you, indianagardengirl and BillMN-z-2-3-4. Yes, night creatures, I get it and our backyard is fenced. What next is the question?! Thanks again.

I might try planting Kalmia latifolia (mountain laurel) shrubs aound those to hide the browsed branches. The laurels are evergreen and deer do not tend to munch on them. That way you can keep the privacy of the arborvitae while enjoying the beauty of the mountain laurel. That is, if they grow where you are. They are native here in Northern Virginia as are the Arbotvitae.

Thank you. Great idea. I am in Long Island, New York and they seem to popular in New England. I will look into that further. It's a smart solve. Thanks again.