People , someone or something rubbed or chewed this “table leg”. Dry rot doesn’t just start falling apart with no abrasion and besides it is painted.

Looks like dry rot, remembering our years when we had a wooden boat

If you look at my blowup of your photo, you will see an opened tunnel and a hole. It looks like the chair leg was made with wood that was already partly rotting and perhaps infested, as with many dead trees—insect kills—cut in state and federal forests. If the wood was wet inside and then painted, the rot will increase, and that can draw insects, some of which only gnaw on already punk or rotted wood. I don’t think this is carpenter ants—too small. It might be termites. Another possibility is Powder Post Beetle, which may have been in the wood for years. Get rid of the chair, definitely, and possibly have your house checked for termites.

I think MandoMan nailed it.
We ordered furniture from a large furniture store here a few years ago. Delivery was delayed several months.
The delivery person told me that the furniture came from China, and they had a truck arrive that was infested with bugs.
Sometimes the termites are internal and take a while to work their way out.

bite marks
 

I'm so very sorry. Apparently my puppy, Teddy, has been visiting you.

Furniture pressure treated? They used to use copper arsenate I believe that is no longer used-not to good for people working with it or people eating off tables treated with it.
In any case the preservative does not go fully thorough every part of the lumber.
When, it is cut to length it opens untreated wood to attack.

You should not sand or burn lumber treated with copper arsenate.

Once again I respond on this site with actual hands on experience. In my construction career I probably built close to 2,500 decks, arbors, gazebos and walkways/ramps using pressure treated lumber and yes it was treated with copper arsenate. We even burned scraps in fires to keep warm and to dispose of the waste. Cut it , sanded it , carried it on shoulders without a shirt on . I’m still ok

It was a rat.

Definitely polar bear.

I was a biologist for over 40 years and have see this exact same damage many times.

A crack addict from Leesburg.

Aaahahahaha! You win! That is definitely the culprit.
😄😃😅