I'd guess voluntary career phase transition. Women is a really good guess. 30 is sort of a terrible age to choose for this sort of thing... Another possibilit

I'd guess voluntary career phase transition. Women is a really good guess. 30 is sort of a terrible age to choose for this sort of thing... Another possibility: lots of high earners are "done but not retired" by 40. They have a big enough nest egg to step off the corporate ladder, but not enough of a nest egg to stop working entirely. So they have part-time income from teaching university courses or consulting, but it's nowhere near their highest-earning years (which wouldn't been in their 30s).