Large corporations like Twitter have been fighting against NSA spying for awhile, although defeats have been far more common than victories. While much of Twitter's data consists of publicly available tweets, a vast amount of personal data is also stored within the company's walls.
If you live outside the United States, our services are now provided to you by Twitter International Company, our company based in Dublin, Ireland. Twitter International Company will be responsible for handling your account information under Irish privacy and data protection law, which is based on the European Union’s Data Protection Directive.
If you live in the United States, the services will continue to be provided to you by Twitter, Inc., based in San Francisco, California, under United States law.
It's also possible that Twitter is making this move mainly for tax purposes. It's become common for corporations to move some of the operations to Ireland in order to save money.