This video seems a little better quality:

https://www.youtube.com/watch?v=aL5eddt-iAo

Almost certainly. The solid fuel would be bonded to the casing of the stage, so little fragments of burning fuel and casing would likely result in generation of thrust, which could causing spiraling fragments.

There's another video: https://www.youtube.com/watch?v=fPHkDc-CwoQ

I expected it to self desturct.

Why would it not do that? Was it to close to the ground?

There wouldn't have been much point in a self destruct. The rocket launched, then it looked like there was an explosion in or near the boosters (which apparently caused a loss of thrust), then it fell back onto the pad and exploded. Blowing it up at a higher altitude just would have spread the debris a little farther.

According to the press conference just now, the range safety officer apparently did send a destruct signal. The signal probably would have been sent very close to the time the rocket hit the ground. In my relatively ignorant opinion, it was going to blow up one way or another. We'll probably get more details later.

Out of curiosity, what does this signal actually trigger?

Each rocket contains conventional explosives which are used to destroy the vehicle if attitude control is lost during flight. It's to prevent the rocket from causing more damage by possibly crashing into a populated area.

http://en.wikipedia.org/wiki/Range_safety

Usually it's a rope of explosives running up the barrel of the rocket. The goal is to quickly remove structural integrity, and the rocket takes care of blowing itself up immediately.

I'd say it was to close to the ground to self destruct. And there would have been a call from the Range Safety Officer in the event he ordered a destruct or the computer called a destruct.

While it probably wouldn't need to be destroyed, the last thing you want is for part of it to lay down on the ground still producing thrust. Better to be certain.

I don't think Antares can command itself to destruct. The destruct has to be commanded by the range. There isn't any point to issuing the destruct command unless the impact point is dangerously far from the flight path, and that wasn't the case here.

I think it's reasonable to assume that Antares can command itself to destruct given that other rockets have that ability, and have so destructed.

Ariane 5 (the infamous flight 501) self-destructed.

http://sunnyday.mit.edu/accidents/Ariane5accidentreport.html

"The launcher started to disintegrate at about H0 + 39 seconds because of high aerodynamic loads due to an angle of attack of more than 20 degrees that led to separation of the boosters from the main stage, in turn triggering the self-destruct system of the launcher"

A SpaceX rocket self-destructed:

http://www.extremetech.com/extreme/188593-spacex-rocket-self...

"In a statement, SpaceX says the rocket detected an anomaly and automatically initiated its self-destruct sequence."

The Titan IVA-20 explosion was also due to a self-destruct, and not the range officer:

http://fas.org/spp/military/program/launch/titan_iv-20_sum.h...

"At this point, the northern-most Solid Rocket Motor (SRM #1) separated from the core booster, initiating the Inadvertent Separation Destruct System. At 45.529 seconds, approximately 3 seconds after the automatic destruct sequence, Mission Flight Control Officers sent command destruct signals to the vehicle."

Up until a few months ago, I was an engineer for OSC. We used the same Flight Termination Logic Unit (FTLU) as Antares. I can confirm that it can provide for automatic destruct should the engineers desire it. I have no knowledge of the specifics of its use in Antares (I did not work on the Antares program), but given the constraints of operating from Wallops, I would not be surprised if automatic destruct is enabled for all Antares launches.

For what it's worth, this mission had a manual destruct command sent at T+20s.

EDIT: According to the press conference.

http://www.orbital.com/LaunchSystems/Publications/Antares_Us... has a glossary item for "Automatic Destruct System".