Tom the only thing I've heard that causes this is woodpeckers trying to get at bugs, but it does also look like it might be a squirrel trying to carve out a new home.
You may have a bug problem that you do not know about.
Try putting some tabasco sauce on the effected areas and see it it stops.
A squirrel might move on while a woodpecker will just peck at another spot.
Woodpeckers do a lot of damage to healthy trees looking for bugs.
My father in law who lives in VT shoots them on site.
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