It is unclear how widespread it is, but the good news is that it looks like whatever it was Google has since addressed it. According to Kevin Kirk, a support manager for Google, he confirms that the issue has since been fixed. “Sorry for the inconvenience, our engineers have found the cause and fixed it. Please make sure that you’ve updated to 10323.65.7 on 65 Stable and that should prevent you from randomly rebooting in the future.”
Google did not detail what the original problem was and it seems to be a relatively recent issue dating back to March, but hopefully the update has rectified it. If you are a Pixelbook owner who has been experiencing random reboots, then you’ll want to make sure that you’re updated to the latest version of the Pixelbook which should address it, assuming your laptop doesn’t reboot halfway while it is updating.