Proper soil moisture is important in caring for a Peperomia. The number one cause of yellowing leaves among Peperomias is overwatering. Only water when the top 50-75% of the soil is dry. Water until water flows freely from the drainage hole at the bottom of the pot. Be sure to discard any excess water that flows into the saucer, as this can lead to root rot and eventual death of your plant.
Your watering consistency
Alternating between bone dry and wet soil from ill-timed waterings can create stress and cause your Peperomia to wilt and potentially yellow.
Weakened or stressed Peperomias become more susceptible to insect infestations. Sap-sucking bugs like spider mites can drain your plant of moisture. This problem quickly manifests itself by yellowing leaflets and fronds. Scale, mealybugs, and spider mites occur frequently in indoor conditions. If not killed early on, these small pests proliferate and move all along leaves and fronds and into nooks and crannies. The piercing mouths of the insects exhaust your plant and accelerate yellowing, especially if your Peperomia is already unhealthy from a nutrient deficiency or improper soil moisture.