One theory is that someone with high level mcdonalds twitter access accidentally posted it while logged in, thinking they were on their own account.
But I'd rule out Robert Gibbs, because nothing in his previous twitter feed has vitriol to it
maybe just a plain jane hacking / password phishing incident

They made an official statement that Twitter told them their account was compromised. I doubt that'd make that up considering the damage if they were found out.

McDonald's haxed, insults Trump

You're not fooling anyone, Russia

Putin must be mad about him trying to build more nukes than them.

it was the hamburglar he thought he was logged on his personal account. Nbd

If it was a rogue employee, they wouldn't be technically incorrect