Why iShares 20+ Year Treasury (TLT) fell 3.88%
iShares 20+ Year Treasury (TLT) fell 3.88% on September 18, 2026 — Weaker mortgage outlook sparks duration selloff in long.
What happened
Market strategists flagged deteriorating mortgage-bond technicals—specifically convexity and duration mismatches—suggesting broader fixed-income stress could ripple across the long-end Treasury curve.
Why it moved
When MBS face convexity headwinds (higher rates trigger prepayment risk that compounds losses), investors often delever duration across the bond complex; long-dated Treasuries absorb that selling pressure as traders…
Why it matters
This is a symptom of the rate-volatility complex: sticky-high rates are now stressing the mortgage market's embedded leverage, and that stress is beginning to bleed into broader Treasury positioning.
What would break the thesis
If rates rally sharply or mortgage strategists confirm the weakness is isolated to MBS with no spillover to Treasury supply demand, the selloff stops and the broader fixed-income stress narrative collapses.