Yellowing leaves are generally caused by a lack of  sunlight, or overwatering a plant. 

Lack of Sunlight -

It is normal for some interior and lower leaves on a plant to turn yellow and fall off occasionally since these leaves are shaded by exterior leaves. A plant will typically only keep the amount of leaves that it has the natural light to sustain. If the plant is losing an excessive amount of yellowing leaves, try moving it closer to a window.

Overwatering -

Yellowing leaves can also be caused by overwatering. Typically when this is the case, there will also be some browning on the leaves. In this case, you should remove the plant from its pot and examine the root system. If the roots are looking brown and squishy, this is root rot, which happens when the soil does not dry out enough between waterings. 

If your plant is experiencing root rot click here to find out more.