Why My Apple iPhone 5s Rebooted automatically?
Without jailbreaking / being a developer with Xcode / etc, it's not easy to find what caused a reboot. If it only occurred once, I'm more inclined to say that there's nothing wrong, but regular reboots can indicate either a software or hardware fault. If the device reboots repeatedly, try restoring the device's OS in iTunes. If the problem persists then that may indicate hardware failure.