Could just be a glitch, or maybe a system file, or your OS updated in the background, although USUALLY you will get notified of this. I wouldn't worry about 2 random reboots just yet. Keep an eye on the device and if it KEEPS happening then maybe its time to deal with it. Could be a rogue app, or otherwise application issue. You might try remove the last app you installed and seeing if this fixes the problem, BUT like I said, 2 reboots is not cause for alarm YET. Keep monitoring.
I'm August update with my unlocked Moto Z Play and I've never had a reboot issue Thank God. Loving this 3+ day battery use. I still haven't needed my Battery mod haha.