The Ultimate Cause: Flight to Liquidity
The ultimate cause of the LTCM debacle was the "flight to liquidity" across the global fixed income markets. As Russia's troubles became deeper and deeper, fixed-income portfolio managers began to shift their assets to more liquid assets. In particular, many investors shifted their investments into the U.S. Treasury market. In fact, so great was the panic that investors moved money not just into Treasurys, but into the most liquid part of the U.S. Treasury market -- the most recently issued, or "on-the-run" Treasuries. While the U.S. Treasury market is relatively liquid in normal market conditions, this global flight to liquidity hit the on-the-run Treasuries like a freight train. The spread between the yields on on-the-run Treasuries and off-the-run Treasuries widened dramatically: even though the off-the-run bonds were theoretically cheap relative to the on-the-run bonds, they got much cheaper still (on a relative basis).
What LTCM had failed to account for is that a substantial portion of its balance sheet was exposed to a general change in the "price" of liquidity. If liquidity became more valuable (as it did following the crisis) its short positions would increase in price relative to its long positions. This was essentially a massive, unhedged exposure to a single risk factor.
As an aside, this situation was made worse by the fact that the size of the new issuance of U.S. Treasury bonds has declined over the past several years. This has effectively reduced the liquidity of the Treasury market, making it more likely that a flight to liquidity could dislocate this market.