I Have Never Had A Credit Card
This is a graph, copied from Megan McCardle at the Atlantic, who copied it from someone else, of the ratio of Debt to GDP. If I had to guess at the circa 1980 action which led to the current explosion, it would be Fed Chairman Volcker's decision to radically(?) change the main tool by which the Fed attempts to manipulate the economy. Previously, the government would print more, or less, money, in order to to grease or slow the wheels of industry. Since then, effectively, money supply has remained fixed, while the Fed has adjusted interest rates (the cost of debt) to achieve the same ends.
One (unintended?) consequence of this is that bond(debt) trading went from the sleepy backwater of "5%" to a market where millions could be made or lost quickly. Some of my understanding of this issue comes from the very readable "Liar's Poker," by Michael Lewis, which, although a bit naive at times, is a firsthand account.