Two words: Cool & Quiet. If that is already BIOS enabled, that means the AMD will automatically underclock to somewhere around 700-900MHz and will turn off the CPU fan because at that level, it runs so cold, it doesnt even need a heatsink in reality!
I will bet this is a possible issue. Either that, or like mentioned above, your fan control in the motherboard is set to regulate the fan speed, and will shut it down. My FATALITY doesnt allow the fan to shut off, not even pass below 1200RPM as a safety feature because of the massive OCing potentials. Beware, if your CPU goes into full load without the fan spinning, it could overheat and damage the transistor series, effectively ruining the CPU itself.