TL;DR: Han was getting in the way of investigating Yoon, the president who had been impeached for trying to enact martial law. Also, they need six out of nine justices to agree to uphold the impeachment but right now they only have six, and Han has been obstructing the nominations of three more justices to fill the empty seats. Nevertheless he’s been pretty cooperative and gotten of the way and the deputy prime minister/finance minister is in charge right now.