This could be for many reasons. From a long hot shower to a few hours of cooking. Excess moisture can easily be created and condensed, even quicker if there is no ventilation and the room is poorly heated.
In more details, moisture is created in the air and funnily enough can get trapped in a poorly heated area- most commonly walls or windows- if not ventilated. This would most likely cause mould patches on internal walls.
For tips on how to avoid excess moisture in your home download our fact sheet ‘How to prevent mould and condensation in your home.