From
The Register - not a top source, but usually not too bad.
Apparently the Uber car had only a single-top mounted LIDAR sensor, which creates a blind spot low and close to the vehicle. Most self-driving vehicles have 5 or 6 including side-mounted sensors.
A spokesman for Aptiv, which supplies other sensors and software, claimed that their systems were disabled on the car in question.
Looks like a case of Uber being Uber and cutting too many corners.