It stopped because it recognized that it needed to stop to let an emergency vehicle get past it. Unfortunately, it stopped right next to a double-parked garbag

It stopped because it recognized that it needed to stop to let an emergency vehicle get past it. Unfortunately, it stopped right next to a double-parked garbage truck in the next lane. No one had programmed in a set of contingencies for "stopped to let something pass, no room for it to pass". This is one of who knows how many contingencies it doesn't have a solution for which is why, in my opinion, this kind of thing will be quite common for a long time until enough events like these are found and software is changed to handle them. I'm hoping that none of these kinds of unanticipated events will cause fatalities, but I'm skeptical that will be the case.