Service outages are not specifically related to Titanfall — which was released today — according to Hryb.
If you are having issues signing into Xbox Live, we are aware of it and actively working on the issue. This is not a #Titanfall issue— Larry Hryb (@majornelson) March 11, 2014
According to the Xbox Live service page, the team is currently working to resolve the problem.
Sign-in difficulties do not appear to be affecting all Xbox Live users.
Update 2: A Microsoft spokesperson provided Polygon with the following statement:
"Microsoft is investigating an issue that may interrupt customer access to some Microsoft online services. This is unrelated to the launch of Titanfall and the Xbox Live service is up and running. The issue on Xbox is resolved, but unfortunately a small percentage of Xbox One owners may still experience a sign-in issue on Xbox Live, which can be fixed with a cold reboot of the Xbox One by powering down, unplugging, waiting 10 seconds, plugging back in and powering back on. The Xbox team sincerely apologizes for the interruption."