In many Pittsburgh area intersections there are not seperate lanes for those who are turning left. (The topography does not allow for adding lanes as we might desire to.) Therefore a person turning left at an intersection holds up all traffic coming from that direction.
So it is customary if there are cars stopped at a red light for the person travelling straight ahead to wave an oncoming left turner through when the light changes to green. This allows traffic to flow in both directions. It's really common courtesy.
Now, this is so common that people turning left will often assume the courtesy has been granted and will go ahead and make the left immediately after the light changes to green. This can startle the out--of-towner, but it really is not a hazard. If you haven't even begun to move your car and the guy ahead of you jumps out and makes his left, it's not like you're going to step on the gas and hit him.
These rules apply when cars are stopped at a red light. Obviously if traffic is flowing through an intersection no one is going to jump out and try to make a left. What they will do is creep into the intersection so that when the light changes to red, they are then free to make their turn, since they are effectively blocking the traffic from flowing in the other direction.
SD
that sounds similar to Mechanicsburg, too. It does take some getting used to.
I feel bad for that woman. I hope she is okay, too. She must feel awful.