The liquidity and trading in those older issues is so tiny that it doesn't give much guidance as to the potential cost for the government of borrowing substantial sums at what are known as ultra long maturities - but for what it's worth the 1932 warloan pays an implicit interest rate of 3.9% at its current price.
The causes of 1990-91 and 2007-09 were similar: Real estate collapse, stock market turbulence, financial engineering turkeys come home to roost, loan failures, taxes going up, a Middle Eastern war, a spike in oil prices and everybody de-leveraging at once.