From what I read it was because Bush froze oil shipments to NK including providing power (which was part of the agreement). Of course also part of the agreement was for NK to stop their nuclear research. So each one just keeps escalating it a notch...
What's kind of funny is that NC voluntarily admitted that they have been doing nuclear research, during their last NK-U.S meeting, and it came as a shock to the administration. The U.S. administration was clueless. Which is why we don't even know where the labs or the nukes are. But instead of us continue talking on this issue, we broke off the talks, cut off the oil and power part of the agreement and the rest is history.