Leaves fall off rubber plants for a number of reasons, such as light changes and over watering or over fertilizing the plant, according to Gardening Know How. Rubber plants can also lose leaves when there is not enough humidity.
Another reason rubber plants can lose leaves are pests. Scale bugs in particular tend to infest rubber plants and cause issues. According to SF Gate, rubber plants naturally lose bottom leaves with age. However, this happens more often with rubber plants that stay outside. Keeping rubber plants in pots that are too small is another reason for dropping leaves. Typically, rubber plants need to be repotted to accommodate growing roots. Failure to repot rubber plants results in roots that become twisted and prevent proper nutrient absorption.