So overvaluation, program trading, herd behavior—maybe some of the possible causes of the crash of 1987?
Dunno. I don’t consider a correction of 10% or so after a runup of 88% over five years to be a “crash”.
There is definitely a herd mentality to markets and they overrun and underrun and correct themselves over time.
Best thing is to invest as much as you can save (20% or more if possible) over decades into a mix of stocks and bonds and just let it be.