Sewers back up because of sewage drains clogging, lack of sewerage maintenance and a number of other reasons contributing to the blocking including flood water. Other not much common reasons of sewage backing up could be a nearby tree roots causing the sewer water to back up into the parts of a building where you would not expect it. The lowest drains in a building would be the first one to suffer a sewage back up. But in short, the obvious reason behind sewage backing up is blocked and leaked drainage.
Sewage drains can become frequently clogged with waste material and other objects blocking the flow of water. This is a relatively minor problem, with local drain clearing solving the problem. Sometimes the main sewerage line can become flooded, which is far more difficult to deal with. This is often the case when a locality is flooded as a result of heavy rains. Due to the overwhelming inflow of water, the sewerage infrastructure at certain points may not be effectively draining it, resulting in flooding and sewage backing up in the buildings located there.
Another reason why sewage can sometimes back up is due to the damage to the drainage pipes. Leakage in the drainage pipes could also cause sewage to back up in a building. Roots of trees can sometimes be drawn to sewage pipes underground. The growing roots can damage the pipes, and due to the pressure, the ensuing leakage results in sewage backing up. However, these reasons are more related to local drainage problems instead of sewers of a locality backing up.
In case of flooding, unless the flood water subsides, there is little chance for the clogged sewerage to become operational again. In case of blockades and damage, remedial actions need to be taken to repair the pipes and to get rid of the blocked waste through various techniques.