Nothing substantial, sorry. If they'd painted it differently I could see moving the cars back to provide a 'bike box' for bikes, but you don't mention that. I think I had seen "far back stops" when I was there; to make life easier for buses sounds plausible, or for fire trucks in one place (I think there's a place on Somerville itself where if cars went up to the corner they would be blocking a fire station.) As for how the intersection worked before... I have dim memories of seeing cars have to back up for a turning bus, so maybe they had just relied on low levels of traffic and thus conflict?