It seems Google has given Twitter some kind of penalty, possibly for cloaking.
Watch this video for the details
Google is not being blocked as there is a cached page, but the cached page is not what is given to users, even when not logged in.
The pages are significantly different in structure.
Just to qualify the value of this video – some people rightly or wrongly look on me as an SEO expert – I don’t proactively consult partially because I don’t believe my own opinions half the time and in the past I have spent a great deal of time trying to prove myself wrong.
Ultimately even Google engineers directly involved with webspam or working directly on Google’s algorithms don’t know every aspect of how Google’s algorithms might work in every eventuality, and some results or side effects may be unintended.