At the height of the past glaciation (about 20,000 years ago), the land was pushed down by the weight of the ice (isostatic depression).  All of the ground-up rock was deposited in the surrounding ocean, which had penetrated significantly inland. The loose deposition of the silt and clay particles in the marine environment, allowed an unusual flocculation to take place.  Essentially, this formed a strongly bonded soil skeleton, which was glued by highly mobile sea-salt ions.
At this point, there was only the formation of very strong marine clay, which is found all over the world and highly stable, but with its own unique geotechnical problems. When the glaciers retreated, the land mass rose (post-glacial rebound), the clay was exposed, and formed the soil mass for new vegetation. The rainwater in these northern countries was quite aggressive to these clays, perhaps because it was softer (containing less calcium), or the higher silt content allowed more rainwater and snowmelt to penetrate.  The final result was that the ionic glue of the clay was weakened, to give a weak, loose soil skeleton, enclosing significant amounts of water (high sensitivity with high moisture content).
Quick clay deposits are rarely located directly at the ground surface, but are typically covered by a normal layer of topsoil. While this topsoil can absorb most normal stresses, such as normal rainfall or a modest earth tremor, a shock that exceeds the capacity of the topsoil layer — such as a larger earthquake, a large mass added near a slope, or an abnormal rainfall which leaves the topsoil fully saturated so that additional water has nowhere to permeate except into the clay — can disturb the clay and initiate the process of liquefaction.