Drains can get blocked for a number of reasons.
One of the most common is due to household waste, such as oil and food waste, that gets washed down the sink.
This causes a minor blockage that gets worse with time, exacerbating the problem and eventually leading to backing up and nasty smells.
As well as waste from the house, leaves and other organic matter can get into drains from outside the house or through gutters. This again causes a blockage where you can’t see it.