One or more investors piled into an exchange-traded fund that's highly sensitive to swings in long-term US bond yields on Tuesday, a day before the Treasury Department unexpectedly ramped up buybacks of long-dated debt and sparked a rally. The surge in trading volume for the iShares 20+ Year Treasury Bond ETF (TLT) or similar long-duration funds suggested advance positioning ahead of the Treasury's announcement.

On Wednesday, the Department of the Treasury revealed plans to increase buyback operations for long-term securities, catching many market participants off guard. The news triggered a sharp decline in 30-year yields, which fell 10-15 basis points, boosting prices of long-bond ETFs.

Traders noted the unusual timing of the large ETF inflows, which occurred just 24 hours before the official statement. While no wrongdoing has been alleged, the pattern has drawn attention from market surveillance teams. The buyback program, aimed at improving liquidity in off-the-run issues, marks a shift in debt management strategy.