Severus and Fleur were the first to arrive at the St. Mungo's, getting the Patronus from Kingsley about the potential threat. It was something that the Goblins would do, attacking the injured and sick for maximum casualties. Severus knew the Ministry had kept stronger defenses around the Longbottoms as a professional courtesy, and knew he could use that to his advantage. As they apparated directly into the protected Ward, he saw Healer Strout with his former colleague Gilderoy Lockhart practicing his alphabet in cursive. Perhaps since we're not on the ground floor nor underground, we have a better chance at fighting off the Goblins.