When federal agencies fail to issue regulations, respond to petitions, approve plans, review standards, or take any number of actions that are required by statute, the federal Administrative Procedure Act (APA) and federal environmental laws authorize citizens to sue the agencies to force them to carry out their legal obligations. Indeed, Congress anticipated that citizens would play an important role in the enforcement of federal environmental laws. When faced with lawsuits for failing to perform non-discretionary duties, agencies tend to settle because their liability is clear.
Stephen M. Johnson, Sue and Settle: Demonizing the Environmental Citizen Suit, 37 Seattle L. Rev. 891 (2014).