This rumor stems from a source in Brussels, Belgium, who spoke to Phoronixs Michael Larabel at FOSDEM, one of Europes larger open source conferences. According to this source, who is presumably one of Microsofts open source developers, Microsoft is taking a meaningful look at releasing a full Linux port of Office in 2014. The sudden change of heart is apparently due to Linux showing commercial viability, and because Microsoft is reportedly already working on Office for Android.
Until now, Microsoft has never released a piece of desktop software for Linux (with the exception of Skype, but that was an acquisition). Microsoft does have a Linux department, though, which has mainly been tasked with maintaining Hyper-V (virtualization) compatibility with Linux OSes under Windows. Presumably, with the development of Office for Android, Microsoft has beefed up its number of Linux developers, and those developers will then also work on Office for Linux.
The other possibility is that Microsoft might be reacting to increased uptake of Linux and cloud-based productivity suites by large institutions, such as universities and governments.