Observation and Condolences
The Sunday morning launch of a Falcon-9 / Dragon spacecraft supply mission to the ISS ended abruptly in an explosion a couple of minutes after launch. My condolences to Spacex over this, and may God speed their efforts to find out what caused this, and to fix it.
We will need them flying again, sooner rather than later, to send supplies to the ISS, and later, to send astronauts there. With both them and Orbital ATK “down” with vehicle explosions, the only other way to send supplies is via the Russian Soyuz / Progress cargo vehicle, which itself has had a fatal problem just a few months back.
The ISS crew has sufficient supplies through to October. After that, they would be forced to abandon and come home in the emergency Soyuz craft docked to the station. Somebody has to be flying and make a successful trip by then. We really do need all three outfits up-and-flying.
Suggestions and Recommendations
If I were them, I'd be looking real hard at propellant leaks in the second stage. Early at lower altitude I saw signs of propellant flash-burning erratically alongside the rear end of the first stage. Then I saw flashes of white "cloud" moving down the side of the vehicle coming from the interstage.
The higher the altitude and thinner the air, the more of this leaking "something" I saw. When the explosion occurred, I saw "for sure" in the video that the first stage was still intact with all engines firing, for a finite amount of time. It was the second stage that exploded first.
That suggests to me that something reactive was leaking from the second stage into the interstage volume around that engine. You need that protected space to have a fume buildup that can be ignited. They were approaching the staging point, and systems were firing up electrically to make that happen. Electricity and loose propellants are an explosive combination.
It couldn't have been just kerosene, there's almost no air up there with which it could burn. You can see that by the first stage rocket plumes ballooning out in the nearly-zero backpressure.
It had to be both kerosene and liquid oxygen that were leaking! Holes in both tanks cannot be ruled out, but that seems most unlikely. Especially as there is no internal path to get leakage from the forward tank into that interstage.
I'd be looking very, very seriously at the propellant line stop valves to the turbopumps (or even the engine chamber itself) that might have opened prematurely somehow.