It's hard to say with such a short description, but as i read it, the google car never left its lane, just moved right to the edge of it's lane to avoid some debris.
It sounded like it was merging into the bus's lane. But I imagine Google will prep some very fancy videos of the incident from the car's perspective within the next week or so, and then we'll know for sure.
You got that backwards. They moved right in their lane to slow down for a turn (while allowing traffic going straight to pass), and then had to move back into the stream of traffic as there was debris preventing them from turning on the far-right side of the lane.
From the writing in the article, I found it extremely difficult to get a good visual of exactly what happened in the accident. I wish the description was a little more, uh, descriptive.
Why did the bus move into an occupied lane?!?