The problem apparently involves MSA, Microsoft's single sign-on service, which authenticates users so they can log into various Microsoft services. Some Azure services were also impacted. The outages primarily affected the U.S. East Coast and parts of Northern Europe, according to Gizmodo, citing data from DownDetector.com.
The same problem occurred just two weeks ago, but Microsoft officials have not commented on the reasons for the authentication issue at that time. So far, there’s no official word on what caused the latest issue either. However in a status update on Skype Heartbeat, Microsoft posted it was "actively investigating problems, preventing users from signing-in and sending Skype-to-Skype messages! We'll let you know as soon, as issues are resolved."
As ZDNet notes, MSA does not use Azure Active Directory for authentication and Microsoft is currently in the process of merging the front ends of the two authentication services. It's possible issues are arising from that effort, though Microsoft has not confirmed anything.
Outages happen to even the largest service providers, but systems going down twice in two weeks could leave some customers soured toward Microsoft's services.
Amazon Web Services has also had some recent outage issues. In February, internet access was disrupted after Amazon Web Services began having increased error rates in one of its East Coast computing regions. That issue was ultimately attributed to human error.
As more companies migrate to the cloud, outages have much bigger impacts. For example, the AWS outage last month took several major retailers offline.