Why North Korea defies order and fires their missile?

Because they have the guts to do so.

North Korea is, and always has been, a rogue nation that doesn't give a flip about U.N. orders. Basically, they refuse to play by the rules. If they can do anything to harm anyone who disagrees with them, they will--no one should be so naive to think that they're all of a sudden going to stop doing what they're doing, unless a bribe is involved, and only then if the bribe suits their fancy. Bill Clinton himself gave N.Korea money (and I dare say some of our technology) to get them to quiet down.

official explanation is that it was a satellite launch but you have to ask how many missiles USA, UK, India, Pakistan, China and in fact most other countries have fired in recent years. North Korea may be considered to be a threat but I can't see it being anything too worry about