She was released when the last grand jury ran out of time. A new grand jury was called, she refused to appear before it and was put back in jail. Presumably there’s an upper limit on how long she can be in jail (this grand jury also has a time limit, plus once Assange’s trial gets far enough along, there won’t be any point to demand that she appear before the grand jury).

Unless I'm missing something, that means her continued incarceration is by her own choice.