McDermott, Pomeroy, Ros-Lehtinen, and Faleomavaega all took trips to Seoul in 2001 or 2003 paid for by an organization called the U.S.-Korea Exchange Council. It was largely funded by a Korean holding company called the Hanwha Group, but run by Ed Buckham of Alexander Strategy Group, who worked hand in hand with Abramoff sharing clients, funneling money, etc. So these trips to South Korea would be well within the purview of investigators trying to piece together Abramoff's activities. And Buckham himself, since he was the key cog in the DeLay machine, moving sums of money about and providing access to DeLay for a fee, would interest investigators on his own terms.
That the Justice Department is looking at these lawmakers doesn't necessarily mean that they're under investigation themselves - more likely they're just trying to put together one piece of the puzzle - but at the same time I can't say with certainty that they're not. DeLay and some others got in some ethics trouble for the trips because they were paid for by a foreign registered agent, which is against House rules. That's not what would interest prosecutors about these trips, though. They're important in the context of the larger Abramoff machine of which lawmakers were a part, sometimes unconsciously, oftentimes fully cognizant of the part they'd been cast to play.
I can't explain prosecutors interest in John Sweeney, the Republican from New York. But perhaps we'll figure that out soon enough.