"This could be the start of a very nasty pullback," he writes on CNBC.com.

Concern about Federal Reserve policy and earnings could help spark a correction, he says. So could worry about domestic and overseas economic growth as well as "Russia's renewed imperialist tendencies."

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Insana cites several other bearish factors.

• "Market leaders tech and biotech have become tech- and bio-wrecks.

• "There has been considerable froth in the IPO market.

• "Economically-sensitive commodities like oil and copper are breaking down, while gold and silver are plunging."

• The market will soon hit a negative seasonal period, he says. "Sell in May and go away.

• Market valuations are stretched."

Still, Insana doesn't think a meltdown is coming. "We've had two crashes since the year 2000, we are not due another one in the absence of true bubble-like conditions," he writes.

Long-term investors should buy U.S. stocks using dollar-cost averaging, he says. "Another running of the bulls should start later this year."

The market's rampant bullishness appears to have eased for now. “People are reducing their risk portfolios a little bit,” John Fox, director of research at Fenimore Asset Management, told Bloomberg.

"Some of the speculative parts of the market have been selling off. If you own a stock and the reason you own it is it’s going up and it stops going up, there’s no reason to own it."

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