What is the reason for computer going into a boot loop if it is switched off from the PSU switch?

Likely reason is an issue with a motherboard itself, or maybe the RAM. If you remove the RAM and disconnect the drives, the system should consistently be able to boot into the BIOS - if not, I'd say either the mobo or the PSU is about to fail.

Reasoning behind: basic troubleshooting. Simplify the setup to bare minimum to boot. If boot is successful, start adding parts one at the time. Once the problem appears, you've found the point of failure.

To identify the actual failing part, new parts are needed. For example if the failure appears when a RAM module is inserted, the problem can be the RAM or the mobo connection. To identify the exact part, test with a different RAM module. If the can now boot sequentially without failure, the original RAM module is the failing part.