A recent disqualification ruling that prevents Kasowitz, Benson, Torres & Friedman from representing InterActiveCorp in a property contract dispute with The Georgetown Co. had an unusual side effect: It revealed why a high-profile family law practice left Kasowitz last year.

But the decision, issued by New York Supreme Court Justice O. Peter Sherwood, was unusual in another way: The judge also disqualified Georgetown’s lawyers from DLA Piper, effectively barring counsel on both sides from representing their respective clients.