Pretty well known that housing values move quite a bit. As the old saying goes in real estate "Location, Location, Location". Land is something you can't import/create, and geography controls where housing can realisitcally be placed. 2008 was a housing bubble, and the US has had almost predictable speculation on land values every 20-30 years of its existance, that leads to a bubble and bust. Used to be cheap western land as the country expanded westward, but now speculation occurs on the current land.
My understanding is a fair number of people got in trouble by refinancing their mortgage on the growing equity of their property, and then getting caught flat footed when the bubble inevitably burst. Also, some states (California in particular comes to mind) have incentives for people to stay in their own homes, which directly lowers the supply of housing, making the remaining properties swing more in value. So if you are not living in the property, you can simply hold it, flip for an even more expensive and better located property, let appreciation/demand drive that price higher, and repeat. Or sell the appreciated property, use the difference to buy two properties (one of value equal to the property just sold, at its original acquisition price, and a second even cheaper property). Basically comes down to land being exclusive use, and non movable.