I don't know if this happened, but I still have a nagging feeling this may have been part of it. Have you ever gone to pull out of a side street, you get an reasonable opening to go and the car you are about to pull in front of guns it? Or you are pulling off the highway, going 60 mph on one of those infamous New England exit ramps where cars enter the highway just ahead of where cars are trying to exit. You are driving along, turn signal on, the guy getting on the highway has his turn signal on, is going maybe 35 mph, is supposed to yield, but decides to whip in front of you?
I really have a feeling these two guys knew about each other. The twin guy was like the one who sees you trying to pull out of a side street and speeds up. The 152 guy like the one entering the highway. He knows what he is doing can get himself killed, but he wants to be first, or whatever the reason, and goes for it.
The bottom line is we can't fly like we drive. We have to actively avoid potential impact all the time, we can't leave it to the other guy to avoid us.
Another point is we may never know if the 152 guy was already turned when he called turning base after the twin guy, in the end it doesn't matter. The thing is, even if he already turned, he could have easily turned BACK to the downwind.
Finally, it appears the 152 didn't fly a squared off base, rather he cheated the base toward the numbers. Had he flown a proper base he most likely would not have collided with the twin as it would have flown by him.
It's just a sad accident.