Cross reference financial information on millions of americans with data breaches from yahoo and linked in, and the social graph data that's freely available from both and you have a serious national security problem. It would be easy to search for employees with serious financial problems at any institution you wanted to target with either blackmail or further intrusions.
Impacting the financial system is like cutting off oxygen to the military machine. And further domestic discontent is a side bonus.
It will be interesting to find out who they think is responsible.