Just got through to the SFO office and had a conversation with their "team lead." I mentioned that I'd been denied and was unaware of any reason why my enrollment would be remotely controversial. She said I could write to the Ombudsperson, but he would simply echo CBP's decision, which was final and "not based on an error." She said my only recourse to determine the reason was to make a FOIA request. She then hung up on me.
Thank God for FOIA.