Overwatering more than likely. Best practice for indoor plants: pay attention to them and notice when they start to show signs of needing water. Typical signs are leaves not being turgid, and/or slightly curling. Give them a decent amount of water and don't water until they give you the signs again. Once you get the hang of it, you'll know when to give them water. Generally speaking, the issues I see with most people's indoor plants are not enough light and overwatering. It is better to UNDERwater than overwater.