My macbook pro troubleshoot reset with grey screen
The first rule of computer troubleshooting: "isolate and identify."

Which basically means undo any changes you made recently. If you plugged a new mouse in, remove it. If you installed a new app, remove it, etc.

In your case I would remove the drive in the optical bay and put the Apple optical drive back in and see if the issue repeats.

User "n1000" hints that it might be related to an OS upgrade or your specific model of Mac. If you can go back to the previous version of the OS that would be good (though time consuming).

You could also take the Mac to an Apple store and have a Genius run some diagnostics and see if it is a warranty or known issue. Note, Apple will blame the SSD in the optical bay so you will have to put the optical drive back in for Apple to diagnose the issue beyond blaming the SSD and sending you on your way.