Following the 2000 Dot Com crash, then Fed Chair Alan Greenspan brought Fed Funds rates down to ultra-low levels. Under 2% for 3 years, and 1% for more than a year.
Rates this low — and for that long — were simply unprecedented. They wreaked havoc with the traditional fixed income market. Bond managers scrambled for yield, and found it in investment grade, triple A rated residential mortgage-backed securities (RMBS). This better interest rate was created by securitizing mortgages with an unhealthy slug of higher yielding, riskier, sub-prime mortgages.