The shape of that graph is roughly equivalent to the shape of labor force participation for women [1]. I don’t think that detracts from your point in regards to the last 20-30 years, but in regards to “before that it was never this high” I think it’s evident that the societal shift of women joining the workforce is the reason, not an improvement in the economy.
[1] https://fred.stlouisfed.org/series/LNS11300002