At T+6ish seconds, while the vehicle is still on the launch pad, you can see a massive explosion at the base of the first stage. Then another engine blows the side of the ship apart shortly after getting airborne (T+29s), then a few more engines go later. It's possible the later failures may have been a result of the big one at the start.
I'm wondering if the fist stage simply didn't get the speed necessary for the second stage to safely separate. *edit: Looking at it again, it appears to me at least, that it started tumbling well before expected separation. With no payload, I can imagine they decided not to separate for safety reasons.
I noticed the same during ignition/lift-off, there was a chunk of debris catapulted way high up and my first thought was an "oh oh!". I wasn't actually expecting it to go too far from the launch pad, was pretty surprised the rocket kept going.
I'm wondering if the fist stage simply didn't get the speed necessary for the second stage to safely separate. *edit: Looking at it again, it appears to me at least, that it started tumbling well before expected separation. With no payload, I can imagine they decided not to separate for safety reasons.