I just applied for my 2nd CitiBusiness AA credit card, after closing the previous one 3-4 months ago (I have had it for about 1.5 years).
You didn't explain: Did you close just the card account, or did you close both the card account and the "master" account?
If you didn't do the latter (which you make no mention of doing), it's not really closed!
The customer service rep said it was because I have had a previous card within 18 months.
Customer service reps at Citi often don't give the real reason. The real reason may only be on the snail mail letter you'll get (in a little while).