Listened to the tape. ATC initiated the sidestep. Listen to it again
Some of the posts that I see here surprise me...
For those that are convinced that these pilots will never do it again...why not, they did it once already. There are people who have made the same mistake twice.
For those that find the Jepp chart confusing, why. It is crystal clear that the taxiway is not a runway and is specifically labelled multiple times as taxiway with separate note not to land on it. None of the runway designators are aligned with their associated runway.
For those baffled as to why this could happen, ask yourself why does an experienced pilot or mechanic walk into a moving prop. It happened to a highly experienced bush instructor pilot this summer in Alaska. Do you think he wasn't aware of the danger. Of course he was.
So one has to make an analysis and ask themselves why. Well, the Alaska pilot had a sudden event happen to him on his sloped terrain with multiple Piper Cubs parked with engines running to keep unexpected swarms of mosquitos at bay. One of the aircraft started moving by itself and the instructor suddenly reacted to this event by forgetting about moving props and running into his own prop while attempting to get toward the moving aircraft.
So what likely happened in Seattle. Likely, a couple of very experienced and locally familiar pilots well aware of this taxiway issue got a last minute runway change as a visual maneuver and the known taxiway issue was completely forgotten about and new important priorities took over requiring a significant amount of new concentration for a procedure not necessarily done on a regular basis.
For those that think something like this, not necessarily specifically this exact event but something along the same lines as this(whether aviation related or not) can't happen to them...you are very wrong.