Certified GKH Gardening Expert
There are several possibilities. If the plant was underwatered or (more likely) overwatered, it could lose leaves. Other forms of stress, such as being in too strong sunlight or being infested with an insect or disease, could also cause leaf loss. Coleus is susceptible to a downy mildew fungus and a virus.
Check for other symptoms on the plant, such as signs of insect damage or discoloration.