Physical injury to a leaf may cause it to turn yellow and drop off. Something may have happened while the particular leaf was forming to weaken it. There are also many environmental causes. Dry soil, overly bright lights or cold temperatures can all make leaves turn yellow. Do not be concerned if a single leaf starts to decline, regardless of its location on the plant.
Light is important because it helps maintain healthy green leaves.
This is especially true for those leaves near the base of the plant. There is a tendency for these leaves to be shaded and they may receive only modest light. When this happens the oldest leaves tend to turn yellow and drop from the plant. Light influences the plant's ability to make food and keep its leaves in good condition. When new leaves appear, older leaves often turn yellow and fall off.