Pothos leaves turn yellow for several reasons, but overwatering is the usual suspect. Pothos can tolerate a lot, but wet roots in low light will still cause trouble.
Quick Answer
Pothos leaves turn yellow because of overwatering, underwatering, low light, natural aging, nutrient deficiency, or root rot. Check soil moisture first.
Overwatering
If yellow leaves are soft and the soil is wet, let the plant dry and check drainage. Pothos do not like sitting in soggy soil.
Low Light
Pothos tolerate lower light, but they grow better in bright indirect light. Very low light slows water use and can lead to yellowing.
Natural Aging
One yellow older leaf is normal. Many yellow leaves at once is a care issue.
What to Do
Move the plant to brighter indirect light, water only when the top few inches dry, and check roots if the plant keeps declining.
Bottom Line
Yellow pothos leaves are usually fixable. Start with watering and light before blaming the plant for being dramatic.