Give me an all expense paid trip to see the house and I will be better able to diagnose the situation. Thought I'd try.

Without knowing your exact conditions, I will offer what I have seen cause this issue.

Occasionally I have seen this happen in moderate temperature high humidity where the paint is unable to dry properly leaving the moisture trapped behind the dried paint film. Also, due to lack of ventilation in the wall, moisture can get trapped behind the dried paint film. I have also seen a particular species of wood which was high in tannin oils cause this as the film dried and then again where the sun shone directly on the siding. Is this an interior or exterior occurrence?