From their blog:
During our scheduled maintenance window we experienced some unexpected heavy traffic at 3a PT. We’ve been working throughout the night to resolve the issues and get back to 100% functionality as soon as possible.
Sounds like a Multicore Crisis. Been a lot of it going around lately!
Related Articles:
As Paul Harvey says, here is the rest of the story. Twitter did have a multicore crisis and rearchitected to move past it.