I'm to understand that Microsoft was one of the companies that had more relaxed interviews in the early 90s, and then took a turn for the more intensive in the mid-90s.

Perhaps the turning point was Jim McCarthy's "beware of a guy in a room", which he meant both figuratively and literally, leading to a transition away from solo or shared offices or even cubicles for software engineers and toward open-plan arrangements. I think that his wisdom led first Microsoft, then the rest of the industry, to treat software engineers not as the darling whiz kids they were in the past, but more a cohort to be closely watched and controlled, as the company is risking a lot on them.