edit: down voted for asking a reasonable question? WTF? I google for "Microsoft sues Linux" and nothing reasonable came up (it's pretty much mostly the Tom Tom stuff, which wasn't about Linux). Hence, I asked what the poster was thinking of.
"whatever the reason might be" as if it wasn't clearly stated in the article.
"In July 2009 Microsoft submitted Hyper-V drivers to the kernel, which improve the performance of virtual Linux guest systems in a Windows hosted environment. Microsoft was forced to submit the code when it was discovered that Microsoft had incorporated a Hyper-V network driver with GPL-licensed components statically linked to closed-source binaries."
So obviously Microsoft didn't do it for altruistic motives.