King has mutant luck powers (what was the name of that guy with 4 fingers on each hand? Hang on...no, not Domino...Longshot! That's the one). But they only manifest when he is scared.

-Saitama shows up when he is in danger (off-screen most of the time) from random monsters and hero-hunters.

-Young Emperor literally made up a power for King to have, which only worked due to remarkable coincidence.

-Saw Tatsumaki's panties, after getting shocked by Saitama's abrupt appearance.

At first I thought he was braver than he lets on (he wandered into the Monster Association alone), but I think he's just afraid of letting people down. Which activates his powers.