The plant is dying of thirst in a sea of plenty. Plants need air in order to take up water. When the soil is saturated, the plant responds as though it's experiencing a drought & sheds leaves. You need to get (and keep) the level of moisture in the soil under control. You didn't mention fertilizing, but too much fertilizer and other salts in the soil solution can contribute to the negative impact of too much water in the soil or cause leaves to shed even if moisture levels are appropriate.

Al