Let's start with this: Obama hates Hillary.
Obama's reason for living is his insatiable arrogance and narcissism. He believes he's smarter, more insightful and better qualified than any underling or any prior president. Which is why he's surrounded by ass kissing sycophants like Valerie Jarrett. No one, not Michelle, not Uncle Joe, not Hillary, not even the dignity of the Office of the President is more important than the chain-to-his-wrist, "guard this with your life" priority of his legacy. Uber alles.
Obama hates Hillary with a hard blue flame. He held he beneath contempt during the 2008 primary, and bought her off, as is common among the Chicago organized crime circle, with those you wish to keep under your thumb until such time as, etc.
He hated her throughout her term as Sec Of State, as witnessed by his damning with faint praise throughout, and his documented (thus far) attempts to marginalize her Neocon warmongering influence, witness Libya, Afghanistan, Russia/Crimea/Ukraine, and Syria. And ISIS.
He hates her now, for being such a senile ditz over her handling of documents critical to the national security including tens of his own emails, let's not forget, which were announced to be in there by the State Department, because they had to explain the presidential privilege reason for not releasing them.
For the third time in this post, Obama hates Hillary, she threatens his legacy, and she puts him in actual legal jeopardy himself.
Only thing I don't get is why he endorsed her.