Murray's always been inconsistent. He has some great patches and some not-so-great patches. It's no surprise he's having a bit of a let-down after winning his first slam. It's nothing compared to his slumps after the 2010 and 2011 Australian Open final losses.
I'd say it's likely that he's mentally fatigued after reaching his two greatest achievements (slam title and Olympic gold) in a single year. And now he has more expectation to win, which can sometimes make it harder. Or after winning those big events he's struggling to find motivation for events like Tokyo. There's any number of reasons, but none of them are surprising.