With negatives already known, why are pros selling now?

Up and down Wall Street investors were attempting to make heads or tails of the sharp selloff on Wednesday.

Both the Dow Jones industrial average and the S&P 500 fell more than 2 percent before paring some of the losses, as market professionals attempted to digest a wide range of catalysts threatening the market.

"The slowdown in Europe, China, ISIS and Russia," said Zachary Karabell, head of global strategy for Envestnet on CNBC's "Power Lunch." "They're all headwinds for stocks."

Traders on the floor of the New York Stock Exchange.
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Traders on the floor of the New York Stock Exchange.

However, Karabell went on to say that all of those negatives have been looming over the market for quite some time. They're not unknown.

Both Karabell and Dorothy Weaver of Collins Capital agreed that the market has priced in those negatives, at least to some degree.

Both pros said the sharp selloff was more likely triggered by something the market hates—the unexpected. And the rapid developments involving the spread of Ebola have been completely unexpected.

In fact, the situation has taken such as sharp turn for the worse, that President Barack Obama abruptly postponed a trip to instead stay at the White House and convene a high-level meeting about the Ebola situation.

Read More Obama to hold Ebola meeting at White House, postpones trip

"Ebola is the one thing that nobody thought of," said Weaver. "It's always the thing that the market didn't think of that becomes the tipping point."

CNBC Contributor Peter Boockvar of The Lindsey Group sees the market a little differently.

He doesn't think it's Ebola; on CNBC's "Street Signs" he said it was QE.

"The last two times we ended QE the market fell 15 to 20 percent," he said. "A $1 trillion stimulus program goes to $0 this month. That is the overriding factor in the weakness."

Read MoreMore QE might be appropriate if economy faltered: Fed's Williams

Whether it's Ebola, QE or something else entirely, Jim McCaughan of Principal Global Investors thinks investors should look at these sharp declines as an opportunity.

Also on "Street Signs" he said: "I think the selloff is technical not fundamental. A rational investor should buy dips in select stocks."

Kimberly Foss of Empyrion Wealth Management said much the same. "If you're a long-term investor and you're diversified, you can add to positions on this selloff." On CNBC's "Closing Bell" she added that although the decline isn't pleasant, she views it as a healthy correction, one that was much needed.

And if you don't have the stomach to buy stocks right now, "Fast Money" trader Joe Terranova of Virtus said on "Closing Bell": "Sit back and let this play out. The market needs time. What happens over the next few weeks will be telling. There are also positives on the horizon."