I see that OTA did a dowel bar retrofit on the eastbound lanes. I don't recall the roadbed being that old; but that's a clue to the soil condition in the eastern part of the state. Much of the same problems on I40 between Checotah and Webbers Falls. Wish I knew some engineering so I could give a better explanation.
Back to the pike, the history of the road from Tulsa to Siloam Springs is fascinating. OK33 before the creation of US412, it was known as the most unsafe road in OK. When I would drive to Fayetteville for the annual TU/UA football game, it took well over 3 hours. Very narrow, poor sight lines, etc. You could buy shirts that said" I Drove 33 and Survived". Thanks should go to the late Dan Holmes who was a driving force in getting funds allocated to improving the route.