2012 Switches to gas mode with Battery Charge remaining

I have a 2012 Chevy volt with 140,000 miles. It has not had any issues until now. The car will switch to gas mode while there is still plenty of charge remaining in the battery (as much as 80%). The temperature is well above freezing. Stopping, shutting of the car off and restarting resolves the issue. I took it to the dealer. however, the computer was not writing any errors codes.

Any ideas what could be causing this?