AP

The biggest gainers in the S&P 500 in 2015 are slumping Monday, worrying some traders. If these reliable winners start flailing, it could bode poorly for the broader indexes for the new year, they say.

Last year, Amazon.com Inc.AMZN -6.73% easily contributed the most points to S&P 500, according to S&P Dow Jones Indices, and heavyweights Microsoft Corp.MSFT -2.64%, Google parent Alphabet Inc. and Facebook Inc.FB -3.15% were the next three biggest contributors. In 2015, Amazon shares more than doubled, Microsoft gained 19%, Alphabet added 47% and Facebook shares rose 34%, while the S&P 500 declined 0.7%.

[EARLIER: 5 Best Performers in the S&P 500 in 2015]

On Monday, all four are lagging the broader market. Amazon, which also received an analyst downgrade, tumbled 5.5% in recent trading, while Microsoft fell 3.2%, Alphabet declined 2.9% and Facebook lost 3.3%.

The S&P 500 was recently down 2.1%.

“Weakness in the beginning of the year certainly stands out—it shows maybe there’s a lack of confidence that those names can continue to perform this year,” says Brett Mock, managing director at brokerage JonesTrading. “They were the narrow leadership last year and people obviously hung onto those positions to year-end, and I think a lot of people were sort of hiding in those names.”

The thinking is if these names can’t maintain their gains, the broader market may struggle in 2016.

Another big winner last year was Netflix IncNFLX -4.71%, which rose 134% in 2015, but is also struggling on the first day of trading in 2016 partly due to an analyst downgrade. Its shares are off 6.9%.

On the other hand, energy-related names such as Exxon Mobil Corp.XOM -1.18% and Kinder Morgan Inc. were the biggest detractors from the S&P 500 in 2015. Kinder Morgan is outperforming the broader market Monday, up 1.1%, but Exxon Mobile is falling, down 1.8% in recent trading.

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