Thank you for asking this question in the support community.
You will want to check the "show version" output to see if there is a core dump message. Also, you will want to check the FLASH to see if there is an exception report that has been created.
Usually, if you are running the latest maintenance or feature release (if not I recommend upgrading the firmware to the current maintenance release listed on the product web page), I would suggest contacting ADTRAN Technical Support to investigate the cause. You can contact Technical Support in the following ways: