A big part of it is simply profit-taking. After huge increases, just a little bit of uncertainty will get investors to sell off part of their stocks to secure the profits. It's always been that way.
The Nasdaq almost doubled in less than a year, so a sell-off of about 5% (!) is supposed to be a big surprise? These things tend to avalanche, starting from just a small uncertainty.

The 1-year Dow looks very similar:

Neither chart accurately shows the drop over the past few days, but the Dow is currently at 10,173 while the NASDAQ is at 2,205.
Bottom line is, neither index has surrendered more than a few months of increases:
