Maybe I missed it but when the booster was supposed to land or begin the landing burn it was lost by the cameras so it probably impacted somewhere other than where it was supposed to land.
It exploded at the start of the landing burn, not on impact. This was the first attempt to re-use a booster.
Booster was not intended to be recovered. They said before launch they were intentionally testing how far they could push it until it exploded so they knew what the true limits are.
The commentary gives me the impression that the booster didn't fail due to the reasons they were expecting it to. I think that they were expecting a successful engine relight and an eventual destruction due to loss of vehicle control.