There are a few possible reasons why your pink Nanouk plant is turning brown. One common cause is overwatering. Pink Nanouk, like other Tradescantia varieties, prefers well-draining soil. If the soil remains constantly wet or the plant sits in soggy conditions, it can lead to root rot and subsequent browning of the leaves. Another possibility is inadequate lighting. Insufficient light levels or too much direct sunlight can cause stress to the plant, resulting in browning leaves. Evaluate the watering routine and lighting conditions to address these potential issues.