Looks like they moved to using the SUIT CSS framework - https://github.com/suitcss. Again the framework is being developed by 3 guys working at Twitter.
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.
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.
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.
Thus, the concept fork and a new framework.