MacBook Pro is crashing

Just to be clear, your Mac isn't just going to sleep as it usually would if you don't use it for a while? The password window would be normal in that event, but you can check your Mac's sleep settings: Set sleep and wake settings for your Mac - Apple Support.

I am not questioning your conclusion the Mac is restarting on its own. If that is in fact happening resetting its NVRAM is justified: Reset NVRAM on your Mac - Apple Support. I suggest doing that anyway. Resetting NVRAM will reset the Mac's sleep / wake settings to defaults (among certain other parameters).

An archived version of Reset the SMC of your Mac - Apple Support contained the following helpful description applicable to that Mac:

1. Shut down the computer.
2. Plug in the MagSafe power adapter to a power source, connecting it to the Mac if its not already connected.
3. On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.
4. Release all the keys and the power button at the same time.
5. Press the power button to turn on the computer. 

Note: The LED on the MagSafe power adapter may change states or temporarily turn off when you reset the SMC.

Personally I'm convinced it was a requirement to have the power adapter connected while you reset the SMC on those older Macs. Apparently it's not a concern for newer ones since the current version of that Support document makes no mention of it. Or perhaps Apple no longer cares about those older Macs and overlooked that requirement in their revised Support document.

Regarding Step 6 above, it has been my experience on those older Macs that the MagSafe LED may or may not change states depending on the particular model Mac. So "may change states or temporarily turn off" is technically correct. It would be the only visual indication of a successful SMC Reset, but if the LED doesn't change state, it does not necessarily mean it was not effective.

By the way you might also want to follow the instructions for Before Resetting the SMC:

1. Press Command + Option + Escape to force quit any application that is not responding.
2. Put your Mac to sleep by choosing the Apple () menu from the upper-left menu bar and then choosing Sleep. Wake the computer after it has gone to sleep.
3. Restart your Mac by by choosing the Apple () menu from the upper-left menu bar and then choosing Restart.
4. Shut down your Mac by by choosing the Apple () menu from the upper-left menu bar and then choosing Shut Down.

The idea is to begin the procedure with the Mac in a known state: sleep, wake, restart, and then shut down.

An older version of that Apple Support document suggested resetting NVRAM is the Mac sleeps or wakes unexpectedly. In fact it also suggested resetting its SMC for those symptoms, so I would do that as well: Reset the SMC of your Mac - Apple Support

Was there a crash or kernel panic report with an offer to "send to Apple"? If so you can copy and paste the contents here. You may need to use the "additional text" icon.

You may also be able to rule out hardware causes by using "Safe Mode" and determining if the crashes occur while using the Mac while in that mode: Use safe mode on your Mac - Apple Support.

Neither one of those suggestions may be entirely conclusive or indicate anything you can act upon.

Neither procedure is harmful in any way.

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