Possibly a malfunction of the Anti-stall device when she was slowing down? Really sad for her, such a talented lady. Could have been a much more serious outcome.
Plenty of internet ranting going on about whether teams should straight-line test on airfields or not, whether there should have been other vehicles nearby, whether or not she was 'good enough' to be sitting in an F1 car in the first place.
I've heard that it could have been an anti-stall malfunction or even that the anti-stall kicked in as a result of her letting the car slow down by lifting off of the gas, and it just caught her out.
What it boils down to is that this was a terrible, freak accident and rather than apportioning blame, we should be wishing her well. Hope she pulls through with no further complications.