Nanda Maharaja was surprised that Yasoda, Krsnas mother, could have bound her beloved child in such a way. Krsna was exchanging love with her. How then could she have been so cruel as to bind Him to the wooden mortar? Nanda Maharaja understood this exchange of love, and therefore he smiled and released Krsna. In other words, as Krsna, the Supreme Personality of Godhead, binds a living entity in fruitive activities, He binds mother Yasoda and Nanda Maharaja in parental affection. This is His pastime.