The cargo ship belonging to North Korea was probably storing products or substances that are banned from being internationally traded. It is also possible that the ship was intentionally sunk. Due to the fact that North Korean is an isolated country with an isolated economy, and exterior merchant activity warrants a degree of attention from the outside world.
When it comes to North Korea, everything is suspicious. There is almost nothing good about that country and it's leadership, and the new leader, Kim Jong Un, is even more evil that his father was. Now, maybe this was nothing out of the ordinary, but with the North it's always going to be suspicious.
Oh, come on! Give me a break. Not everything has to be suspicious or weird! I am starting to become sick of all this conspiracy theories about every little thing that goes on in the world. Come on! Why do we have to doubt the original sources of information nowadays?
The North Koreans are often known to be smuggling illegal weapons and drugs through their ships and if they refused to comply with the authorities then it was okay for them to be attacked, I don't think that there is any conspiracy or anything sucpicious about that in the end.