From the ACLU's press release:

The lawsuit was filed on behalf of a group of prominent journalists, scholars, attorneys, and national nonprofit organizations (including the ACLU) who frequently communicate by phone and e-mail with people in the Middle East. Because of the nature of their calls and e-mails, they believe their communications are being intercepted by the NSA under the spying program. The program is disrupting their ability to talk with sources, locate witnesses, conduct scholarship, and engage in advocacy.

The complaint is available here. The causes of action are the Fourth Amendment, the First Amendment, the Administrative Procedure Act, and a general right against the operation of executive actions alleged to violate the separation of powers.