The answer to 'why is my hardwood floor moldy' is that excess moisture is the most common cause of mold growth on hardwood floors. Mold can develop due to leaks, high humidity, poor ventilation, or flooding. Symptoms include discoloration, musty odors, and visible mold patches.
To address the problem, you'll need to identify and fix the source of moisture. This may involve repairing leaks, improving air circulation, or using a dehumidifier. If the mold has spread significantly, it's best to call a professional cleaning service to properly remove the mold and treat the affected area.