Jeff Gundlach, founder of DoubleLine, told CNBC's Scott Wapner Monday that he thinks the odds of the 10-year yield falling back to its all-time low have increased to about 30 percent because of the big short position. The 10-year hit 1.39 percent in July, 2012 and Gundlach said at that time, he thought the odds were 10 percent but he is now concerned if there's a big move in the market toward lower yields, those shorts would cause a massive squeeze, driving yields even lower.
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Traders have passed around plenty of explanations for the recent move lower in long-end yields. A flight to safety due to tensions in Ukraine was one factor, and there was also a lot of talk about pension funds, locking in gains in equities and moving proceeds to the bond market.
"It's a story where the explanation fits the facts," Rosenberg said, noting the pension story is convenient but difficult to prove. "The actual amount of activity to support that story is the missing link," he said. Much like Sasquatch, "you've got foot prints but nobody's seen him."
George Goncalves, head of rate strategy at Nomura, said there was nothing new in the talk about the pension funds, and that does not explain activity in the bond market. However, he did say one motivator for pension funds to move money into bonds may be the result of a change in the way their premiums are calculated by the Pension Benefit Guaranty Corp. They will pay a variable rate premium based on the amount of unfunded vested benefits, and some funds may have been lightening up on stocks because of it.
"Equities have gone up. That helped their funding status so these pension funds don't want to lose that when equities rollover," he said.
But Goncalves also agreed a big factor in the market has to do with positioning and the big short in the bond market. As for the 10-year yield, "15 basis points is Ukraine, and 25 basis points is structural demand, and the fact that everybody is short," he said.
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Goncalves said he is currently neutral on Treasurys. "We're kind of neutral…I would buy on a dip because I think we have another leg lower before it's all over," he said.
Richard Bernstein, CEO of Richard Bernstein Capital Management, said if the move in the bond market were geopolitical, the U.S. dollar would be moving higher.
"What I think is going on is growth has turned out to be a little slower than people thought. There's no inflation, and I think everybody's decided that the long end of the curve is a great place to be," he said.
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Bernstein said he's still bullish stocks, even though is concerned about weak earnings growth. He said the flattening curve would not necessarily be a negative for stocks, but the current move is a worry to the market. "That's one of the reasons the market is not performing now," he said.
What to Watch
International trade data for March is reported Tuesday at 8:30 a.m.
There are also dozens of earnings, including UBS, CBOE Holdings, DirecTV, Discovery Communications, Rowan Cos,Northern Tier, HollyFrontier, Mosaic, FirstEnergy, NRG Energy and Vulcan Materials before the bell.
Disney reports after the close, as does Activision Blizzard, Agrium, Allstate, Electronic Arts, Whole Foods, FireEye, Groupon,Potbelly and Zulily.
—By CNBC's Patti Domm. Follow her on Twitter