Manually shifts on own while in drive

Absolutely no idea why that would happen -- I'm sure it is difficult to replicate, which is of course quite a challenge. If I was guessing, it would be with something related to the transmission mode selecting operation, something losing power and then restoring it, something causing a default. I would stay in touch with your Chevrolet dealership who should reach out to General Motors, and with their vast technical resources and field experiences hopefully will have another vehicle somewhere doing the same thing that they have seen and fixed.