I do not think they will give any other reason if they have not given any now.
I have a strong feeling that her CPT or study time has found to be a violation due to recent changed USCIS policies.
I am not sure though until you really the reason. I am just guessing.
I think you should apply again to see if it goes through. Also, you can contact the embassy (if they have given any contact information in the email) to know the reason.
On a second note, what is your H1B status? Has it seen some kind of issues in past? Any status violations or denials?