It could be that North Korea is harvesting plutonium for nuclear weapons from the fuel rods. Or it could be that they want the US adminstration to think that. Or perhaps there's a technical fault. Or...
Previously, a shutdown at Yongbyon has tended to damage NK-US relations, because of the first possibility.
Discussion on North Korea Zone (I should say that I am really glad that some of these people don't run US foreign policy. On the other hand, some of the comments are interesting).