Excerpt from this article:
But combine that with the fact that the computer was in charge in 22 of those 28 rear-end crashes, and you have reason to believe that the AVs are doing something that makes cars behind them more likely to hit them. Maybe that’s driving herkily-jerkily (as we experienced in a Cruise car in San Francisco in November 2017), or stopping for no clear reason (as we experienced in an Uber car in Pittsburgh last year). That’s not necessarily a bad thing. It indicates a conservative focus on safety: Better to stop for a fire hydrant than run down a preschooler. But part of being a good driver is behaving in a way others expect, which doesn’t include constantly stamping on the brakes.