This was the first flight of the Anatres Launch Vehicle using a new Russian built engine.
The vehicle and the Cygnus CRS automated cargo spacecraft were carrying just under 5000 pounds of supplies to the International Space Station, along with a whole bunch of "cubesats" for deployment
The engines are shipped to the US, and modified by Aerojet for the launch vehicle.
I've watched the video several times (and watched "other" videos of it), and it appears to be either a nozzle or perhaps a turbopump failure. If you watch the exhaust plume, you cab see a significant brightening and change of shape right before the vehicle explodes.
This is a major setback for Orbital Sciences, and the loss of almost 5000 pounds of supplies and experiments for the ISS.
And here's a very interesting aerial view: