My ignorant as all hell take is that this is tied to the collapse of the enforced nuclear family social construct. Women grew their careers and were able to have financial independence. As women began to have the option to choose not to have children many decided to take that approach. Once the decline really sets in it's during the wealth extraction of the middle and lower classes by Reagan and the destruction of leftist and minority family units through the War on Drugs in the '80s and starting in the '70s respectively. By the time the '90s roll around the trends are entrenched and here we are.