Low interest rates and innovations in the mortgage market are to blame
You are simply letting a person to buy more of any big, expensive asset. And the thing does gets out of control fast when there is a even minimal expectation of appreciation, and inflation fears.
And that on top of the fact that real estate is the only "investment" possible for an ordinary person without employing an officeload of securities attorneys in many countries.
Chinese housing market is the prime example (2B vacant square metres, with even more in the pipeline)