Defiance: Where does it come from?

Question:

Dear Jane:

My daughter is 7 3/4. This morning, she left the house and walked to school without saying a word. What is an appropriate disciplinary action for this? I feel that the issues are:

1) Anxiety: in me & her Kindergarten Brother – every morning we walk to school as a family, she snuck out the back door and headed to school. Meanwhile, my son and I were calling around the house, and looking around our property for her. We walked to school hoping that she would be there. My son was worried that Madison would be afraid if we’re at school and she was still home. I knew she would be there, but in the back of my mind I did have worry about her safety.

2) Defiance: I believe part of her sneaking out without saying anything was an act of defiance. Earlier she had hit her brother and taken his Pokemon cards away. I had previously told her not to touch his cards while they were getting ready for school – we have a rule about no playing with toys while getting ready or they get taken away and put in my room. After the incident I asked her to give me her Pokemon cards – which she said were lost. I then asked her to think about what else of hers I could hold on to until she figures out how to stop hitting her brother and playing with cards while getting ready.