See what happens in a case like this, is typically there are two high line feds to different areas (Primary and Alternate), but that also have backup ties. Something must have happened where the two main feeds went down for an area, say detriot (just an example), so another highline connected now to ottowa tries to pick up the load and it switches over. However that line is now trying to support ottowa and detriot, and if the load is too high, the line will trip out to physically prevent itself from burning out. Now Ottowa will be picked up from its back up, which can't handle the load and the whole system will trip out as a chain reaction and all the power plants shut down for protection causing the outage that occurred.
Basically no one has heard what started the chain reaction, but as far as I know, there were supposed to be safeguards in place to prevent a domino effect. Some of them worked, which is why Massachusetts stayed online. Our tie was tripped automatically from Canada at 4pm right when this happened and we stayed on our primary source and weren't feeding back to NY. If that system had failed and didn't trip out, MA would have been out of power too.
Just so you guys know, this is a very general description of what happens, the system is much more complex.
But the finger is pointed towards Ottowa, for the general suspicion of where the chain reaction started. But don't take my word on it, its just speculation by our VP's...