Electricity networks in most parts of the country are the overhead type. This exposes the network to severe weather conditions (wind/rainstorm).
Rain in some cases is preceded by wind. The wind usually carries flying materials such as branch of tree, or flying conductoring materials which may bridge 2 or 3 phases of a 11kV or 33kV line that supplies power to an area. This makes the line to trip from source in order to safeguard the power transformer that feeds that line. A device is incorporated at the source that indicates severity of the fault, this gives an idea to the operator on duty on whether the line can be restored or not.
However, in most rainy weather conditions, a tripped line is not restored, until the rain subsides or the line is patrolled for fear of having a downed conductor or uprooted trees that may have fallen on the lines or broken poles on the lines due to the rain storm.
Another reason is when we have a weak insulator (usually porcelain) that cannot be detected by the human eye under dry weather conditions. This weak insulator remains an insulator but as soon as it gets moisturized by rain water it becomes a conductor and grounds the line and trips it from source.
The exact location of this weak insulator is very difficult to ascertain. Therefore, even when the line is patrolled, nothing may likely be seen but when the line is restored, it usually stays.