When writing your first review, the Nickname field doesn't give any indication that it is going to be permanently set for your account after that point.
I suggest four options to correct or avoid this type of problem for users in the future:
Allow users the ability to change their Nickname.
Add text right next to the Nickname field that says something along the lines of,"This will be your Nickname for any and all future reviews. It is permanent and cannot be changed."
Don't include the Nickname field in the same container as the review. Perhaps this means that the first time a person goes to complete a review, a prompt comes up asking them to set their Nickname that will be used for all their reviews. Having the Nickname in a separate container/view from the review will help users understand that the nickname is not set on each review, but set once for all reviews.
Have users create a Nickname for each and every review. Thus the Nickname lives with the review and is not set on the user.