I got the feeling last night that he may have doubts about Lannan. He wanted to push him and see what he's made of (having a short bull pen may have contributed as well).
I think it was actually McCatty's call. Johnson kept asking him questions during that inning and it looked to me like McCatty was telling him to leave Lannan in. It was Johnson's first game, so it's not surprising that he was getting some help.