The New York Times reports that the ACLU sent a request for information on the tracking program to the DOJ last November, but have yet to receive any documents. The suit is a means to speed the delivery of that information, according to this ZDNet post.
At issue is the bypassing of warrants by U.S. attorneys to acquire mobile phone location information without probable cause. The two groups believe the public deserve an explanation of this practice and whether the DOJ is violating their constitutional rights as a result.
Controversial surveillance legislation that would grant telecommunication companies immunity for their participation in the government's domestic program is also awaiting a Senate vote.