1. Rapid Growth
Plants prioritize their newest, youngest growth. So when the plant doesn't have enough access to nutrients in the root zone, it will take from the older leaves to provide for the newest growth.
If you know you have plenty of nutrients in the soil, make sure they're accessible with adequate water!
2. Improper Watering
Lack of water in the root zone makes it challenging for plants to absorb necessary minerals from the soil. Too little or too much water can interfere in this process!
Remember that the Chinese Evergreen likes its soil to dry out a bit between waterings. It should have moist soil about 3-4" down.
When you see yellowing, that is Nitrogen, and other mobile nutrients leaving the lower leaves to provide for the new growth. This is called Nutrient Recycling.
3. Too Little Light
Plants that don't have enough light self-prune off leaves they don't need. Plants are efficient organisms and when they cannot support a full set of leaves, they will often self-prune lower leaves to make space for leaves higher up.
Their goal is to get as close to the sun as possible! And that means growing leaves UP!
4. It's Just How it Grows!
Pileas grow a stalk up similar to a palm tree!
They shed their lowest leaves to provide for the newest growth. As they grow overtime, a bark-like stalk appears with the scars of where old leaves use to be!