There was just a press conference where he went into some detail, with the caveat that it's all still very early information.

Basically he said that 2 of the 3 engines didn't start up correctly, and caused the booster to hit the water at a few hundred miles per hour.

I'm guessing the dive into the water was purposely missing the barge when it detected the problem, as they have spoken about this capability in the past when they were first making attempts at landing the rockets.

He also said that as long as they have the footage, and it's cool looking, they will post it somewhere!