Kind of normal I think, at the companies I've worked for, most outages were caused by a change in production, for which the impact was not properly assessed.
Also just load. I'm sure many github services are closer to capacity / logjam at 11Am on Tuesday than at 3AM Saturday.
it's because people work (and use GitHub) a lot more on weekdays. A lot of outages are load related.
Almost as though it's load and not code causing the issue.