Call your local sewerage authority or public works department to find out if your connected downspout empties into the sanitary sewer. The problem is too much water. Sanitary sewer systems can only handle a certain amount of water. During a rainstorm, water gets into the sewer from connected downspouts and other sources. When there is too much water for the system, the excess has to go somewhere, and that somewhere is often somebody's basement, a manhole, or a nearby creek or river.
Municipalities have a legal requirement to stop water from overflowing out of sewers.
Even if the water does not overflow, it's still a problem, because the wastewater treatment plant has to treat the extra water. Sewage overflows are messy, costly, and a threat to health and the environment. When there is a sewer backup into a house, the homeowner may have to pay the cost to clean up, repair damage, and replace ruined carpets and furniture. Raw sewage contains microorganisms that can cause diseases such as hepatitis, giardiasis, and gastroenteritis. Long term environmental damage.