I have no idea why it is doing it, but in either position you are turning on the a/c compressor.
I'm not sure if this is even possible, but maybe the a/c compressor is locked up. You can check this by taking the wires off the a/c compressor. Now when you switch the knob to a/c or defrost the a/c clutch won't lock. If it still does it, you know it isn't the compressor.. You may have to just work your way backwards to find where the problem is.
~Mark