A.J. McLean is standing by his fellow Backstreet Boy ... saying there's no deep mystery behind Nick Carter's arrest -- dude just got drunk.
A.J. was on San Diego's "Kramer and Geena Mornings" Friday, and said he feels bad for Nick ... but mostly because the media's making too big of a deal about the Key West bar brawl.
His take is Nick made a silly mistake after a few drinks ... something he says everyone, himself included, is guilty of at some point. Check out his advice for Nick.
All the BSBs always have each other's backs -- excluding Kevin Richardson for about 6 years.