Probably way too early to tell even that. There will be hundreds of possible causes that will have to be narrowed down; no one will know anything concrete for a good while most likely, even inside Orbital Sciences.
You most probably meant inside Orbital Sciences Corporation. Antares is just the rocket.
Argh, I do that every time. Quite right.
In crashes like this there often isn't enough wreckage to reassemble to tell one thing or another. But there will be lots of data to comb through which will probably lead to the cause.