Since Waymo is very heavily California-biased at this point, a possible explanation for this, and one I think is responsible for a lot of rear-end collisions all across the world, is the "California roll." For those unfamiliar, in California (and, again, almost everywhere) it's super typical for people to approach a stop sign and not actually come to a complete stop. Instead, the driver slows down and then continues to "roll" through the stop. Many drivers here in CA expect the driver in front of them to do the California roll and so their calculation of when they will reach the actual stop sign/line will be wrong if they assume the car in front of them will roll.

I've said this on here many times before, but one of the reasons I love riding in Waymos is because they (in my experience) obey all traffic laws to the letter of the law. So if there's a stop sign, they all stop.

Would love to know the specifics of these rear-end collisions because I'd bet that they're either California rolls at stop signs or doing the same roll behavior when turning right on red traffic lights.