Delta Air Lines flight 845 stopped abruptly during pushback at the Seattle-Tacoma International Airport (SEA), Seattle, Washington, resulting in a flight attendant falling and receiving a serious injury. According to the operator, the captain was the pilot-in-command and the first officer was the pilot monitoring. The airplane was pushed back to the gate with the push-tug after the event. According to the flight crew, as the first officer read through the pushback checklist, the configuration and status of the hydraulic panel was missed by both crew members. The airplane was pushed back by the push-tug, and the captain set the parking brake as the first officer was starting the left engine. As the tow bar was removed, the ground crew asked the flight crew to verify that the brakes were set, and the captain responded that brakes were set. Shortly thereafter, the ground crew and the first officer stated that the airplane was moving. The captain stated he then glanced at the brake pressure gauge and saw no pressure. Both crew members then noticed that the hydraulic panel was not set, and hydraulic pumps were not on. The captain immediately turned the hydraulic pumps on, while maintaining full brake pressure. As the brake system pressurized, the airplane abruptly stopped. The sudden stop caused several of the flight attendants in the cabin to fall. One flight attendant suffered a serious injury to her arm, and another suffered a minor injury.