I'm no expert, but I'd chalk it up to a combination of Linux gaining popularity with the techies (once it became accessible for mere mortal nerds in the early 2000s) and Apple saying "Unix? Yeah, we'll take some of that," and going from a pretty, user-friendly OS buried under 17 years of cruft and kludge to a pretty, user-friendly OS completely re-tooled around a full-featured, time-tested architecture (after some, uh, initial issues, I'm told.) It takes a lot to make a behemoth like Microsoft rethink their game, but suddenly having two credible competitors, one for each end of the market, going up against a pile of feces like Windows ME and a company reputation for developing software as solid as a Soviet-bloc compact car will do it.
All versions of OSX prior to 10.5 were absolute dog faeces tbh.
