Left a little too long
Water is supposed to be our homes in a controlled fashion. It is not supposed to be out of control within our homes and when it does it can lead to worsening conditions if left alone.
Some homes can have chronic problems with ground water that accumulates and routinely comes in. Sometimes it is not known, and sometimes it is ignored. Both situations lead to the same place.
Water starts to cause the growth of microbes such mold and bacteria within as little as 72 hours or 3 days. When a water damage strikes you, time is critical to saving as much as possible and keeping your costs to a minimum.