This is a common occurence in a humid environment, where the local cabin temperature is lowered to dew point. and the cabin doors are open to atmosphere.
If a jetway was connected with an air conditioned environment, the dew point would not be reached.
As a comparison, dew point in Kuala Lumpur is typically 25°C and in Johannesburg -4°C.
I use to see this misting regularly in East London and Port Elizabeth in the days before jetways.