1973 was the first year of the oil crisis. Inflation increased radically, so inflation adjusted rates went negative. Mortgage rates adjusted once they realized that the new inflation wasn't a temporary change.
The early 1980s were when the Federal Reserve increased interest rates to end the stagflation of the 1970s. The interest rate increase pushed mortgage rates up. The inflation decrease stopped hiding that increase. Then afterwards, interest rates went down. So did mortgage rates.
You didn't ask about the 2007-2010 period, but a commenter did. Note again that these are inflation adjusted rates. So it doesn't show the 6% rate. The inflation adjusted rate was lower. It dropped a couple percentage points. Then there was the 2008 deflation. That made inflation-adjusted rates jump briefly. Then they went back down as inflation went positive again.
Inflation adjusting these kinds of numbers can be confusing. Inflation changes can exacerbate or hide interest rate changes. The nominal numbers might be easier to understand in this kind of graph. Sometimes, it's the inflation rate that is causing the graph to change rather than the mortgage rates.