I sort of assumed that Leiter moved to South Florida to be closer to "the drug scene". The CIA does investigate drug running, does it not, so it would make sense that the CIA and DEA would collaborate on a case.
CIA wouldn't get involved with most drug runnings though. That's usually DEA and FBI territory. Still, I imagine the CIA would work on the occasional case. Most likely if Leiter is CIA, it was a request from the DEA. Whether they requested Leiter specifically or just asked the CIA for help, who knows?