Editor asked me why to change the corresponding author I have one accepted manuscript waiting to be published. During the proof, I changed the corresponding author from me to my postdoc adivsor. Editor now asked why I made such a change. 

What happened was that when I submitted my manuscript, I am the corresponding author by default. I failed to find any option to change it. So I planed to change it later on. 

What shall I respond in order not to be rejected during this stage? 

I am major in the theoretical relativity physics. 



Update: 

After nearly one week, my request finally is approved. I do not even have to fill out a form.
Just say what you said here: you intended the corresponding author to be one person, but accidentally set it to the other.  That's a fairly unexceptional reason to change and I wouldn't expect it to raise any major issues.