When flooding occurs many things happen. When the treatment plant gets flooded they stop taking any additional solids from sewer mains. A gravity flow sewer line runs 5 lbs. of pressure per square inch under normal conditions. When the main lines plug and stop, the smaller sewer laterals such as the one running out of your home build pressure. The 5lbs psi can go to 280 psi in an instant.
Sewage finds the path of least resistance.
All sewage at that pressure can go up one service line, sewage pressured up from over 100 or 1000 or 10,000 homes at higher elevations than yours. Sewage from that many homes could rocket up your line when no ground water is anywhere near your residence. Your yard could be dry, with raw sewage half way up your interior windows, add ground water and your conditions get even more serious. This is why every home in America needs an Elder Valve. Valve on all new construction projects.