Why is kotler transferd?

The reader is meant to understand that Mother and Lieutenant Kotler are having an affair, but that possibility doesn't even cross Bruno's mind. When Lieutenant Kotler is suddenly transferred away from Out-With in Chapter Sixteen, Bruno notes that "there had been a lot of shouting between Father and Mother about it late at night," but it doesn't occur to him that Father has discovered Mother's affair with the young soldier and sent Lieutenant Kotler away as a result.