Sump pumps are designed to pump groundwater and rainwater. Sanitary sewer pipes are designed to carry sewage, not groundwater and rain water.
Normal sewage flows often fill the sewer main more than half-way already. So if two or three neighbors in a block illegally connect their sump pumps to their sewer lateral, the flow that is trying to get through the sewer main will be more than the capacity of the pipe. The sewage in the pipe is going to start backing up
It is critical that sump pumps discharge to the yard or storm sewer, not to the sanitary sewer. It is illegal to connect your sump pump into the sanitary sewer, and it can cause serious problems!