Unfortunately the chart only goes back to 1990, and I don't have time to dig up historical figures. Perhaps more financial instruments used to enjoy a AAA rating until the Savings & Loan scandal of the late 80s, and the sudden upward jump in ratings represents the purging of bad assets from the financial system following the recession of the early 90s. But really, I don't know. Without disputing your observations above, I think we could learn a lot from looking at the largest discontinuity here rather than focusing on the absolute volume figures from recent years, notwithstanding the significant increase in securitization following the repeal of Glass-Steagall.