If your leaves turn yellow and fall off of the plant, there are a few reasons behind it. For one, your plant could be subject to too much light; secondly, the temperatures may be too cold, or third, your orchid may be home to spider mites. Your course of action should depend on the cause itself. If the leaf is in a place that is too bright, consider moving it away from that spot. If your plant is in a colder area, consider placing it somewhere a little warmer. Finally, if you feel that the plant is subject to any disease or infection, remove the leaves from the plant. Just be careful when cutting leaves because you could give the infection a greater chance to spread.