I know people who changed jobs for a salary lower than their current one. Sometimes people just don't enjoy what they are doing and they would rather work at a place they love.
Also, while some people will move to a good new opportunity for less money, I imagine it isn't super-common until you get into money is mostly about keeping score level. Many/most people have a lifestyle that's somewhat aligned with their salary do dropping it by 20% is probably fairly significant.