I checked this out [1] just to get a sense, and apparently there have been 200 total collisions involving Waymos, about one every 100,000 miles. 43% were at relative speeds of under 1 MPH. Only 23 were serious enough to cause an injury or deploy airbags, and of those, 16 were rear-end crashes. At least one of the 16 involved a driver who hit the Waymo a second time as he tried to run away, so probably not good faith. Anyway: might be that Waymos hit the brakes a lot, but does not seem like an epidemic of fast "slam on the brakes" rear-end collisions.

[1] https://www.understandingai.org/p/human-drivers-are-to-blame...

A waymo behind the waymo wouldn't have made that mistake.