Why People Keep Rear-Ending Self-Driving Cars

This paragraph midway through the article sums up their guess:

...the computer was in charge in 22 rear-end crashes, [so there is] 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), or stopping for no clear reason (as we experienced in an Uber car in Pittsburgh). 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.