I believe it was because he failed to return to Sweden to serve his Pirate Bay sentence and instead remained in Cambodia where he was living at the time. There was an international warrant out for his arrest and when he was deported back to Sweden he was judged at risk of flight or further “criminal activities”. He was removed from solitary after a few months, so I’m not sure if he was put back there for his later, longer sentence of hacking.

EDIT: He was later held in solitary confinement in Denmark for at least 10 months while awaiting trial for hacking.