could also be people who have moved from XP to a new Windows, and have now moved off Windows entirely to another OS
Not quite, certainly some have gone that way, but it's curious that the upward trend in Windows XP is almost matched by the downward trend in the others.
If it were users going from Windows 8.x to non-Windows, I'd expect the Windows XP trend to remain flat and the bump to show up in either the MacOS X or Linux trend lines. (Of course, there's *BSD too, but they don't show a trend line for that family.)