...perhaps/likely as a result of being ignored by Apple's last round of security updates...
Another possibility is that the shift reflects a bit of pent-up demand, i.e. Snow Leopard users who had already planned to purchase new Macs or upgrade older ones, but who were waiting for a more stable Mavericks release, which 10.9.2 (theoretically) was. (I say theoretically because I waited for 10.9.2 to upgrade from Mountain Lion for that very reason, and don't have the real-world experience to comment on how stable 10.9.2 might be compared to earlier versions of Mavericks; it certainly isn't what I'd call "flawless" at this point, especially the much beleagured Mail.app.)
