In my folks case, it was a stuck on/off button due to crud in it.
“In my folks case, it was a stuck on/off button due to crud in it.”
Story about Samsung phone ...
Phone was low on battery and kept rebooting. Internet searches indicated stuck button. I took the sim card out to start a disassembly. It booted up without the sim card and began charging. After it was charged, put back in. Happened again, same fix.