User blog comment:Krauserlols/Just noticed: most recent champions join the league because.... Morgan freeman..../@comment-1330314-20131004061940

The lore's been toned down in LoL for quite some time, even with the new events accompanying each champion. Every new champion has some sort of motivation, though, even if it isn't made clear in their description:
 * Aatrox fights for the sake of fighting. It's tied to his essence in some way, and he thrives on battle. Since the League is the only place where open combat is authorized, it's the perfect place for him.
 * Lissandra is competing against Ashe and Sejuani for control of the Freljord. Since they're both backed up by the League, she can't declare open war on them, so she's trying to gain power by playing to the League's politics.
 * Kha'Zix hunts the strongest beings he can find, hoping to devour them and absorb their strengths. Every champion in the League is among the strongest on Runeterra and even beyond, therefore it's his ideal feeding ground.
 * Hecarim, Mordekaiser and the other denizens of the Shadow Isles are orchestrating a plot to invade Runeterra. The League is the center of nearly all political and magical power there, so it would make sense that they'd enter it and find out its strengths and weaknesses before launching their invasion.