The theory is simple and is justified as the following, quoted from the document linked to at the top of this page:
Replacing the windows is likely to increase the airtightness of the dwelling. If ventilation is not provided via a mechanical ventilation with heat recovery system, then increasing the airtightness of the building may reduce beneficial ventilation in the building.
In these circumstances, it is necessary to ensure that the ventilation provision in the dwelling is no worse than it was before the work was carried out.
The forum is awash with threads about mouldy walls, excessive condensation and other internal damp problems.