It appears very much like an inability to orient the rocket as desired to get to stage separation - it seemed like the control system was hunting for the correct pitch, heading, and velocity and didn't get there. Perhaps this is either an Ariane V88-like software failure, possibly a sensor issue (which one was it that the IMU acceleration sensor was installed 180 degrees out?), or whatever engines were critical to stopping the reorientation weren't operating.
It's always a slight chance that crap got kicked up during the initial lighting to damage wiring or fuel/oxygen lines or nick some other critical piece. Sure, once all are lit and fully running not much will come back, but that startup is still violent.
I am very impressed that the structure held together through that maneuver. Most vehicles can't stand more than a couple of degrees off-axis before the column collapses.
I'm waiting for Scott Manley for details.