Well, the supposedly "temporary" ultra-low interest rates that were the "solution" for the Crisis following the 2008 Crash only really postponned the day of reckonning.

Further, those and other government measures rewarding Asset Owners (read: the rich) on the backs of those whose Income comes from Work, caused inequality to explode, which means more "unused money" around (every additional €1 of income for the wealthy goes almost entirelly into savings, every additional €1 if income for the poor and working class goes almost entirelly into consumption) and all that extra money was seeking investments, any investments, which was visible in the markets by lower interest rates on treasuries (for example, the famous negative German bund interest rates), lower bond yields (meaning higher prices for the same level of risk), higher stocks and so on up the risk scale, a pumping up of lots and lots of extra investment money which ultimatelly inflated several bubbles at the same time.

Further, the ultra-low interest rates also made it cheaper to leverage one's investments (i.e. invest with loaned money alongside one's own money), further adding money to those bubbles given the way money is created in the modern monetary system (mainly created by banks when they give loans, with only a small percentage actually created by central banks).

This, by the way, also explains things like the crypto bubble.

I really only mentioned the biggest ones I'm aware off, but I bet we'll have a lot of bubbles of all sizes exploding as soon as the first big bubble blows and causes a credit crunch and investment retraction.