I've learned that when dealing with the FBI/NICS check there is often no apparent rhyme nor reason to why 'delay's happen. Often, it's been supposed that an applicant's name is similar to another prohibited persons, but I've never had that question answered 'professionally' when I've been on the line with them.
(I should note that IAW the 1974 Privacy Act, NICS is not allowed to discuss why a particular individual is in delayed or denied status. I should also note that the FBI isn't capricious about categorizing a background as 'delayed'. They have reasons, they just won't tell me.)