This is source I found from another site, main source you can find in last paragraph
A crack has finally formed in the foundation of the U.S. bull market. Now investors must decide if any structural damage has been done.
This year’s hottest stocks, companies from Facebook Inc. and Apple Inc. to Netflix Inc. and Nvidia Corp., buckled Friday, spurring losses that sent the Nasdaq 100 to its biggest drop relative to the Dow Jones Industrial Average since 2008. Accounts of what spurred it ranged from bearish tweets by a short seller to a cautious note from Goldman Sachs Group Inc.
In its most benign interpretation, the selloff was merely a rotation, counterbalanced by rallies in industries such as banks, energy producers and retailers. But the reversal was enough to spur soul searching among bulls who have watched the market value of Apple, Alphabet Inc., Microsoft Corp., Amazon.com Inc. and Facebook increase by $500 billion since December.
“We are probably going to see additional selling pressure on some high-momentum stocks that have spearheaded the rally,” said Chad Morganlander, a money manager at Stifel, Nicolaus & Co. “Stocks have become overbought.”
Even with the decline, the Nasdaq 100 remains up 18 percent in 2017, more than twice as much as the S&P 500, and trading at a significant valuation premium. The index’s price-earnings ratio was 26.1 as of Friday’s close, more than four percentage points above the broader gauge. That’s the widest gap in more than a year.