The other day SpaceX released more details on what probably happened with the DemoFlight 2 launch. Ultimately, the 2nd stage motor shut down prematurely… because it wasn’t getting enough fuel… because the rocket was oscillating… because the fuel was sloshing… because there wasn’t enough damping in the control loop and because there was an impact between the 2nd stage nozzle and the first stage at staging.
You can actually see much of this in the video of the test flight if you watch closely. I recall noting that the staging seemed to be fairly misaligned, and it sure looked like an impact at the time. I was too excited by the cheering from mission control* to pay much attention though. 
Bottom line: The sloshing and the control loop will be easily fixed. The angular rates at separation will have to be looked at more closely I think. They believe they can control this by throttling back the Merlin, 1st stage, motor. This may lead to reduced reuse. I speculate that this may be due to increased pintle wear in the nozzle. Its fairly impressive that their Kestrel, 2nd stage motor, nozzle is tough enough to take a blow from the first stage and still operate pretty much normally. Obviously, having parts of your space vehicle bouncing off of each other is to be avoided.
The details can be found on Elon’s latest update at the SpaceX update site.
* The cheering was audible on the live webcast (along with some amusing remarks after the initial launch abort which are not included and might want to be avoided in the future) and is on the video. In re-watching the staging in writing this post, there is a pretty obvious collision between the 1st stage and the nozzle, and a significant attitude correction shortly thereafter (watch the position of the 1st stage in the background change). Also note the continued rotation of the 1st stage in the background to see how much angular rate the system had before separation (conservation of angular momentum and all that).
