The four-day rally in U.S. Treasuries is threatening to upend the legion of investors who are betting against the longer maturities.

Open interest in 30-year notes -- a measure of positions outstanding -- dropped for a fourth day on Monday, suggesting traders who have taken bearish directional bets are feeling the jitters. If the gauge continues to show a drop, it could herald a full-blown squeeze in Treasuries.

Open positions in 20- to 30-year futures saw a combined reduction of $7 million per basis point, a measure of interest-rate risk exposure being reduced. The cut in positions, equivalent to just over $8 billion of 10-year cash bonds, may just be a start of what’s in play.

Prior to the latest rally in Treasuries, speculative net short positioning was growing, according to data from the Commodity Futures Trading Commission. Investors had more than 280,000 short contracts on 30-year futures as of Nov. 27, an increase of about 30,000 from a month earlier.

Traders would be aware of a strong seasonality in the five to 30-year Treasury spread in December -- which has contracted nine times in the past decade.