Four more possible reasons:
one is to check the cpu temperature with your finger. If it’s very hot cool it.
Two: could be your operating system files or SD card or whatever is corrupt. Prepare a new SD card or download the system image again.
Third possible reason is that you connect something to your pi that makes it fail: USB device, IO pin device, serial device shorted LAN cable,
And last one: the power cable connector are dirty/rusted/loose and the power connection is bad. Usually at power up current consumption is high at certain stages and a poor power connection may drop the voltage and cause restart again and again.