Someone like Ron Johnson is a "retail guy" not a "tech guy". I say that in reference to what his professional passion is. He's also a highly successful executive. The ultimate goal for someone with his abilities is to be the CEO of a major corporation. An opportunity presented itself and he seized it, simple as that. It's a logical move.
If we want to read into it we could, though it's probably just baseless speculation. Johnson was considered by many to be a legitimate candidate to succeed Steve Jobs as Apple's CEO. It is safe to assume that it was clear to him that he wouldn't ever assume that role at Apple, so it was time to move on.
If that's the case we could ask ourselves why did it become clear that it wasn't going to be him? Either a formal successor was announced internally, or Steve is recovering from his illness and is set to return.
At the end of the day, I don't think we'll see any major changes with Johnson gone. He's brilliant I'm sure, but a good executive builds a team that can operate seamlessly without him in place specifically because of scenarios like this one--and Johnson is better than "good". Everything will be fine.