IOS or Android device? New or old?
Likely that phone shut itself off to preserve the batteries, since it couldn’t find a signal.
After all of the complaints about short lived batteries, not surprising to see phone makers become aggressive to preserve a batteries life.
Or, not sensing a signal, the phone shut down so that when it was discovered, there would still be some battery life left to start a reboot.
Pretty logical conclusions. Tks.
iPhone XR, 4 yrs old
Li-ion batteries have to be protected from over-discharging or they become dangerous and damaged, also they are kept partly full to protect your data and settings.
The phone will turn itself off before hurting the battery.