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!
I previously thought that the middle core on a Falcon Heavy was expendable. So it not landing (while needing 3 engines) makes sense, as it was only a week or two ago that they tried a fast/hard landing with 3 engines over open water (as they weren't sure then that it would work). I would expect this landing mode to need at least a couple more tries to get working reliably.
Well that test was unrelated. They only reason they tried that with an ocean landing was because they wanted the barge for the FH demo flight.
This should have been a 1-3-1 style landing burn where it lights up one engine, then 2 more to slow it down, then drops down to just 1 again at the end. In this case it sounds like the the outer 2 engines didn't start up right, so it came in way too hot.
They also have said that the center core of FH is basically a new design from the ground up, so it makes sense that they had some problems on it's first landing.
The booster's trajectory points to water until pretty shortly before landing. Precisely so that when things go wrong the expected result is to miss the drone ship (or landing pad).
