Looks like they moved to using the SUIT CSS framework - <a href="https://github.com/suitcss" rel="nofollow">https://github.com/suitcss</a>. Again the framework is being developed by 3 guys working at Twitter.<p>It may be an extension to the concepts of Bootstrap - but as @mdo has already moved out of Twitter - expecting BS to always be aligned to Twitter would have been more to ask for.<p>Thus, the concept fork and a new framework.
I think they're in their right to evolve and explore. But I think Bootstrap is not an style, it is just a foundation, so, it doesn't matter what foundation they use, what it matters it is the result of the design, wich, imho, lacks in design taste.