It's not unreasonable for there to an expectation of basically zero accidents of this nature during testing in cities. The public puts a huge amount of trust in private companies when they do this. And, pragmatically, Google, Uber, etc, all know that it would be horrible publicity for something like this to happen. One would think they'd be overly cautious and conservative to avoid even the possibility of this.

Lastly, the whole point of the human operator is to be the final safety check.

You're right that we have no idea of the cause until the data is analyzed (and the human operator interviewed). Yet, my first thought was, "Of course it'd be Uber."