I don't know, I think his professional progression makes perfect sense. He broke into this thing by writing, and starring in, a hilarious cult-classic, "Swingers". From that success, he got a gig directing the next movie he wrote, "Made". That one wasn't so great, but appearantly, he made enough contacts to direct a Will-Ferrel comedy, "Elf". That's not unheard of. That movie was a big hit, so now he's a commodity as a director. Gets the opportunity to direct a children's adventure, "Zathura", and does a terrific job with it (while showing that he can direct action). After all that, it just so turns out that he grew up a major fan of the "Iron Man" comic.
Perfect fit.