I'm always telling people in the shop - taking care of plants is a science project! Lots of trial and error, lots of adjustment till we find something that works!
Here are some possible reasons, but you will know best what the conditions are in your home, not all of these will apply.
Moisture stress: Over or under watering
Weirdly, under AND over-watering can result in similar symptoms, so it takes some further investigation to figure out which could be the problem. If your plant's lower leaves are turning yellow and the yellow is spreading fast, it may be because you're giving it too much water. Overwatering means the roots can't breathe, so they stop carrying water and nutrients to the leaves. This can cause the plant to suffocate and die.
BUT, overwatering doesn't just mean you're watering too much. It means you're watering too often. You should water your plant thoroughly each time, so the soil is evenly wet, and allow it to dry out a bit between waterings. If you're not thorough and some parts of the soil stay dry, the roots in those areas can die, and then rot.
Strangely, overwatering can also cause your plant to wilt, and slow down its growth - sometimes this makes us think it needs MORE water and the downward spiral continues!
Yellow leaves caused by overwatering are limp and floppy.
Under watering is easier to fix - give your plant a small amount of water at a time, a few days in a row until it has rehydrated, and it should bounce back if you've caught it in time. Avoid giving it a massive soak, as this can push it into root-rot territory if some of the fine roots have died off. Yellow leaves caused by under watering are crispy and dry.
Yellow leaves can also show that nutrient levels are low. You can fix nutrient deficiency by repotting your plant with new, fresh potting mix, (in spring or summer) or adding a balanced plant food. Indoor plants appreciate being fed all year round - they don't go as far into dormancy as outdoor plants do. Think of it as plant vitamins.
Lack of light
Not enough sunlight can cause a plant's leaves to turn yellow and fall off. Plants are always trying to keep a balance between the energy they receive from photosynthesis and the energy they uses to maintain leaves. If the plant isn't getting enough energy from sunlight, it will prioritise its resources and drop some leaves. This happens gradually, and the plant takes nutrients out of the leaves before it lets them go. The plant will stop dropping leaves when it gets back into a good balance of energy used/energy gained
Leaves that are already very yellow are a lost cause - they won't turn green again but it's worth leaving them attached, the plant will consume all the nutrients left in them before they drop naturally. If you correct the cause of the yellowing - future leaves will be green and healthy.