Incredibly High Fan Speed Reading
There's quite a few possible reasons why fan speeds vary, but as a starting point you're probably best to see whether a reset of the System Management Controller (SMS) changes things for you.

To reset the SMC on your iMac


Shut down your computer
Unplug it from power
Wait 15 seconds
Plug the power cord back in
Wait for an additional 5 seconds
Press the power button to turn it back on