What The Seagull suspected earlier turned out to be more than imagination. Seagull Reference was possibly the first to point to abusing of CALEA backdoor to be the cause of Google's outrage. Today there is proof.
Computer World cited 'a source familiar with the situation' to reveal an 'internal intercept system' of Google, which spy on its own EMail data to comply to legal requests from authorities. No wonder they were able to pinpoint China, because Chinese must be using their affiliated accounts when they wandered around. This echoes the description of the alleged 'comprise' of the said accounts: only account info and EMail titles, but not EMail contents.
In other words, Google's stun was a combined result of embarrassment, rage and an urge of denial.