1) Rising interest rates. 2) China slowdown 3) Increase in unemployment 4) Banks tightening lending practices.
People thinking they are too expensive....
It has started mate. Ask anyone in US 2008 what will pop the RE bubble. No one knows all was rosy.
It does not need to pop. The "pop" comes from prices being way up from whee they should have been. Too many vested interests and spruikers around. May the crash burn many greedy.