After 10 years of near-constant central bank interventions to prop up markets and make stocks, bonds and real estate rise in price -- while also simultaneously hammering commodities to mask the inflationary impact of their money printing from the masses -- it's difficult to imagine that "they" will allow markets to ever fall again.
This is known as the "central bank put": whenever the markets begin to teeter, the central banks will step in to prop/nudge/cajole the markets back towards the "correct" direction, which is always: Up!
It's easy in retrospect to see how the central banks have become caught in this trap of their own making, where they're now responsible for supporting all the markets all the time.
The 2008 crisis really spooked them. Hence their massive money printing spree to "rescue" the system.
But instead of admitting that Great Financial Crisis was the logical result of flawed policies implemented after the 2000 Dot-Com crash (which, in turn, was the result of flawed policies pursued in the 1990's), the central banks decided after 2008 to double down on their bets -- implementing even worse policies.