Update: Microsoft's PR agency reached out to clarify that the lawsuit is not over a failure to pay, but merely a failure to pay on time and with accrued late payment interest. This lawsuit just got a lot more boring.
According to re/code, Microsoft filed a lawsuit against Samsung today for failure to pay royalties as part of a contractual IP licensing agreement between the two companies. The agreement in question is for Microsoft's infamous smartphone patent portfolio, which has pretty much every Android OEM in the US paying out a percentage of their handset sales to the struggling Windows Phone maker.
Samsung Allegedly Stopped Paying Microsoft Its Android Royalties, Microsoft Responds Predictably (Read: Lawyers) was written by the awesome team at Android Police.