This happened because Zheng Kuan, the son of Zheng Chenggong, had done something. This thing is that Zheng Kuan is a bit unruly, but he hooked up with the wet nurse of his fourth son Zheng Rui. After fooling around with Zheng Chenggong behind her back, it wasn't long before the wet nurse gave birth to a child for Zheng Kuan. This incident can be said to make Zheng Chenggong very angry. As an influential man at that time, he was ruined by his son. So Zheng Chenggong ordered his son Zheng Kuan and his wet nurse to be executed together.
What makes people angry is that Zheng Chenggong blamed Zheng Kuan's mother for not teaching him well. So, together with Zheng Kuan's mother, he added a crime of improper godson and killed her. It can be said that the death of Zheng Kuan's mother is very unfair, and it is reasonable to say that this matter has nothing to do with her.
I don't think Cai knows that Zheng Kuan did this. As a mother, if she knew that her son was doing this, especially if he had such a relationship with his grandson's wet nurse, she would definitely stop it, but she didn't do it. At this time, she obviously said that she was an ignorant person.