The photo just looks like a minor collision with very little visible damage. One robotic Prius ran into the back of a regular Prius.
Google’s Prius struck another Prius, which then struck her Honda Accord that her brother was driving. That Accord then struck another Honda Accord, and the second Accord hit a separate, non-Google-owned Prius.
Striking a car with enough force to trigger a four-car chain reaction suggests the Google car was moving at a decent clip. Google says its unable to provide us with a copy of any official accident report, but that may be the only way to know what happened for sure.