Why is my CPU fan running constantly. EVEN WHEN THE UNIT IS OFF!?

Why is my CPU fan running constantly. EVEN WHEN THE UNIT IS OFF!?

When powering down the (A)TC-780A the CPU fan stays running, or worse, ramps up to 100% and stays running. I want to reiterate, this is with the unit off. The power light on the front is dark. Clean install of Win10, all drivers and BIOS updated. Any help appreciated.