As noted, housing bubble, the bubble was started by lack of investment opportunities when people start to amass wealth in cash, housing is pretty much the default choice. Bond and other safe investment do not yield enough to cover high inflation.

It is then further furled by a combination of speculative investment and local government/bank/construction industry collusion. This is quite similar to what happened to the States in post-2000 housing bubble, but in greater scale due to stronger government control and growing economy.