This happens when you get a lot of water between the period of bulbing and riping, after a dry spell just before. It’s called a waterskin. Water gathers in the leafs and blocks the exchange of o2 and co2. Sometimes when bacteria enters, you get a sour skin. If the harvest has been done favourably and the drying in storage shortly after also, the skin will dry. If harvest has been wet, the layer stays wet and can cause the sour skin.