this is what the service centre told me. If they sensors believe its too humid, the auto defogging will come on. But, this never happened last winter in the same car. Some settings have changed and its rubbish.It'll force the climate control on after a while as well.
I was cruising on heated seats only and found that it'll start up the climate control by itself and then put it in defogging mode even when there's no fog.
My guess is they can read cabin humidity directly and have to infer it based on temperature readings. It does seem to get it wrong a lot of the time.
We need an option to turn off auto defogging.