My guild likes to complain about maintenance Tuesdays especially when it gets extended. What are some frequent or common reasons why that might be the case (so I can beat them over the head with it)? I have a few guesses but I never know if it’s actually true.

There's lots of internal minutiae that could cause an extension, but most of them fit into two categories: 1. Processing lots of data. 2. Bugs found during last minute tests.