The most obvious answer is, of course, that someone pushed the power button. Possibly accidentally — e — e.g., if the power button is unfortunately sensitive and someone bumped it slightly while walking by. This can also happen with a failing power button (e.g., springs have worn out). Accidental power button presses can be helped by fashioning some sort of Molly guard to install over the power button.
Asking for a graceful shutdown from, e.g., IPMI will often also register as power button pressed.
Another possibility is (electrical) noise making the system think the power button has been pressed. Make sure the wires to the power button are firmly seated on the board (and the switch, if not soldered) and check cable routing to make sure the wires are away from anything with large/variable power consumption.
If nothing else works, you can edit
/etc/systemd/logind.conf and change
ignore (then restart systemd-logind). That should stop the system from shutting down, but it does mean that you won't be able to use the power button to initiate a clean shutdown (the hold-for-4-seconds unclean shutdown, if any, can't be disabled via systemd as it's done by the system firmware).