Common issue, posted about many many times. Nothing you can do but not harmful, just somewhat unsightly. Combination of overwatering or overly moist soil and too much shade, so the plant simply drops leaves. Fungal issues/diseases can also accelerate the issue, whereas you'd notice more browning and crisping.

The issue can vary greatly from year to year due to variables such as amount of sun, humidity/airflow, rainfall, fungal epidemics