A Hyundai spokesperson told The New York Times that brake fluid may cause the brake control unit valves to corrode, which reduces braking power. Affected cars will have their brake fluid replaced, and if necessary, will have the control unit replaced, as well.
The original NHTSA investigation was launched after the government agency received 23 complaints from owners citing reduced braking power and increased pedal travel. One incident reportedly resulted in a crash, while another caused a driver to lose control of the car. NHTSA has not officially announced the recall as of this writing.