Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
I had heard about the latest Gartner report claiming that Microsoft Windows will become the dominant platform for "Open Source" (and free) software in the future. While there are certainly a number of reasons why some FOSS has and will continue to be written that also runs under Microsoft Windows, I think the fundamental premise is wrong.
Since FOSS software is licensed in ways that enable others to modify and port software to other platforms at their choice, it is very natural that such software will migrate to platforms like Microsoft Windows, and tend to become more cross platform in nature overall, over time. Some software is deliberately designed portable from the start, as this offers technical advantages. The most important technical advantage is perhaps having different platforms and different compilers expose underlying bugs that might otherwise get missed. This is just one way that software freedom tends to also result in higher quality code, and one of the more indirect ways that it happens.
Another reason for seeing more FOSS software under Microsoft Windows is the rise of portable and platform neutral runtime environments, starting with Java, and more recently with things like Portable.NET, Mono, etc.