The Maria Hertogh riots happened because of religious tensions, but also because of ethnic and colonial reasons.
The spark for these riots was the final decision by a court in colonial Singapore that Maria Hertogh should be returned to her biological parents. She was born to Dutch parents but had been raised for eight years (during and after WWII) by Muslims in what is now Indonesia. A dispute over her arose after the war and the court finally ruled that she should go back to her biological parents.
The riots erupted in part because there was a religious element to them, with the crisis being portrayed as an instance of the conflict between Islam and Christianity. However, it also had to do with ethnic tensions (white vs. Malaysian) and with colonialism as it was a decision rendered by a colonial court at a time when nationalism was arising around the region.