A short while ago, an Antares rocket launched from Wallops Island exploded shortly after takeoff. The launch was supposed to be an ISS resupply mission, and had been scheduled to launch yesterday, before being canceled as a boat intruded into the downrange safety area.
At this point, we don't really know what caused this. It will probably be months before we do. Still, I'm going to engage in a bit of idle speculation.
From the video, it looks like the rocket broke apart very suddenly and spectacularly, and didn't deviate much from its course beforehand. Since it detonated extremely close to the pad, it's almost certain that this wasn't an activation of the range safety system, which would be used to destroy the rocket if it had suffered a failure in the guidance system.
Compare the Antares explosion to a recent failed Proton launch:
This article states that the crash was caused by a shutdown of one of the booster engines. The Antares rocket has two engines, compared to the Proton's 6, so in the case of an unplanned shutdown, it would have fallen back to the pad almost vertically. That it exploded in midair suggests that something caused the propellant tanks to rupture and ignite. In my opinion, the most likely thing to cause this would be some sort of overpressure event in the engine. Numerous faults in the engine could cause such an event, and I don't know enough to even make a guess at what it could be.
The Antares rocket uses a single AJ-26 engine in the first stage. This engine is better known as the NK-33, a development of the NK-15 engine used on the Soviet N1 lunar rocket. These engines were all built during the 1960s and 1970s, and subsequently refurbished by Aerojet for use in the Antares rocket. I'm certain that Aerojet has very robust quality control and inspection procedures, but it's not impossible that some microscopic defect could have slipped by. Additionally, until it was surpassed by a variant of SpaceX's Merlin engine, the NK-33 was the kerosene/liquid oxygen fueled engine with the highest thrust to weight ratio in the world; roughly 137:1 (the RD-275M had a slightly better thrust to weight ratio, but used UDMH/N2O4 propellants). That an engine with such performance was developed in the 1960s is a testament to the skill of Soviet engineers. It also suggests to me that the engine was very 'hot', basically squeezing everything possible out of the design with little room for error (I've also heard this about the RS-25). While this would allow higher performance, it (in theory) means that the engine could be more prone to failure. I'm not going to go so far as to say that this is definitively the cause of the launch failure, but it's my guess at this moment.