Not surprised at all. Christie wanted to get back some relevance, after Trump stole his balls. What better way than to be floated as chief of staff, then turn down Trump. Small thing to get back a small amount of his dignity (sorry, the balls are long gone).
What we are seeing at this point with Trump is that people in his orbit are only out for themselves, and if they can pick up some press, prominence off of dogging Trump, all is fair. Trump is deflating rapidly, and there is much less risk in sticking a shiv in his back. Trump has so many enemies at this point, and so much risk of jail, that he can now be crossed.
Christie may be crooked, and self centered, but his is not dumb. That is more than can be said about 99% of the people in Trump’s administration at this point.