I doubt their accounts were hacked directly.
Somebody either breached some third-party API posting service or the cell carrier these people used, hence resetting their 2FA methods.
Logically, if something was breached on Twitter's side, they would have just shut the whole site down rather than risk the fallout that's currently occuring.
Twitter shares have down 5% ATM despite doing well earlier today, so I guess investors aren't happy either way.
A lot of unconfirmed rumors that a twitter employee with access to the user management panel was hacked. But I find that hard to believe. I guess well just have to wait for official announcements.
Whatever it is, it's almost certainly isolated to Twitter.
If it was the cell carrier that was hacked, then we would probably be seeing similar messages posted on Instagram and other popular social media channels.
Also, the fact that only certain accounts seem to be affected, indicates that the attacker is limited in whose account they can post from.
For example, Barack Obama (number 1 twitter user) was hacked. But Katy Perry and Justin Bieber (number 2 and 3) were not.
This indicates there is some commonalities between breached accounts. I suspect it was actually a third party API or their email provider.
