The main signs were a housing bubble, rising mortgage delinquencies, and growing stress in banks and shadow-banking institutions that were heavily exposed to mortgage-backed securities. The main trigger was the collapse of the U.S. subprime mortgage market, which turned into a broader liquidity and confidence crisis after mortgage losses spread through the financial system.

What was building up

Several warning signs were already visible before the bust:

  • U.S. housing prices had surged for years and then peaked in 2006, a classic bubble pattern.
  • Lenders had issued large numbers of risky mortgages, including loans that relied on continued home-price gains and easy refinancing.
  • Financial firms were deeply tied to mortgage-backed assets, so rising defaults quickly threatened balance sheets and funding markets.
  • Early-warning research later found signs of stress in trade and broader financial connections beginning in 2007, before the worst of the crash hit.

What set it off

The immediate trigger was the subprime mortgage breakdown : as borrowers started defaulting in large numbers, the value of mortgage-linked securities fell sharply, and investors began doubting which institutions were exposed. That loss of trust caused funding markets to freeze, turning a housing correction into a systemic financial crisis.

Why it spread so fast

The damage spread because the risk was not isolated to homeowners; it was embedded across banks, insurers, and investment vehicles that had bought or guaranteed mortgage assets. Once those assets were questioned, lenders pulled back, credit tightened, and the broader economy weakened, which is why the bust became the Great Recession rather than just a housing slump.

In one line

So, the signs were excessive housing speculation, risky lending, and hidden leverage , and the trigger was the subprime mortgage collapse that shattered confidence in mortgage-backed assets and the institutions holding them.