Law enforcement believe the activity, which makes it harder to then unlock the phones, may be due to a potential update in iOS 18 which tells nearby iPhones to reboot if they have not been in contact with a cellular network for some time, according to a document obtained by 404 Media.
There’s nothing listed on the Apple Support site about an automatic restart feature when there are no cellular networks nearby. I’m guessing they left the phone connected to a charger long enough to install a previously downloaded update.
When automatic updates are enabled, iPhone will coalesce the software update download with other download activity. The package will remain downloaded and install once the iPhone is connected to power, and remains powered on and locked for an extended period of time. The install initiates a reboot.