That is astonishing. Seems to happen time and again though. But at what point, what trigger makes people think "hey, lets all get in at the worst possible price we can-it just made a new high, thats always worked in the past!!"
There could be a lot of tax selling in the new year. It could be quite tempting to take profits and sit on cash and not pay tax for a year
For anyone who has had money in the market for the last 3 years, why the hell wouldn't they sell those stocks that have given them 100% plus returns? I mean, there are many stocks that are up 300% and more.