There are some quite strong laws in Denmark about the foreign ownership of land, and tax starts to ramp up quite quickly as soon as you purchase a second property. People don't tend to put their earnings into multiple properties like they do in, say, the UK (where I'm from).

I haven't read much into the property market here but I would guess the rapid increases in prices is down to increased demand from people moving here, rather than land owners re-purposing their property. I could be wrong though :)