It doesn't happen often, but it can be telling: managing partners or other law firm leaders abruptly jumping ship from their own firm for a competitor.

Milbank's hiring of Irell & Manella's former managing partner earlier this month is the latest example of a firm leader decamping for a rival. Other examples in recent years include leaders at Fried, Frank, Harris, Shriver & Jacobson; LeClairRyan; Crowell & Moring; and Montgomery McCracken Walker & Rhoads exiting for other law firms.

In contrast to when firm leaders move in-house to clients, sometimes these unusual moves are a signal of a law firm's health: the firms underwent pressures in the years leading up to a leader's departure, such as lateral exits or lower revenue, profits per partner or lawyer head count.