What are things that would cause the CPU frequency setting in BIOS to spontaneously change on its own?

My friends PC stopped booting this am so she had me look at it.. it just wouldn't boot.. CPU fan spun for a second as did the Power Supply fan but then it just sat there.. no beeps, no monitor, no anything.

My first inclination was the PSU or maybe the MBoard. I had a spare PSU and switched it out but it made no difference.

Nothing I could do with the MB except look for blown caps and make sure everything was seated properly. All was ok
with it too.

On a hunch I took out the CMOS battery to test the voltage and it was what it should be but in taking the battery out I also reset the CMOS to factory and then when I booted once more this time it actually got into the BIOS and stopped... the message was it had started in safe mode because the CPU freq was improperly set. I set it to what it should be (which is what was showing now by default) and rebooted and lo and behold all was fine.

So, the question is what would cause that?

tnks.