Donald Trump and Dennis Rodman have been friends ever since Rodman was on Celebrity Apprentice and the Donald fired him over a spelling error. And Dennis Rodman is a friend of Kim Jong-Un. Kim likely released the political prisoners at Dennis Rodman's request.
On a side note: I watched a documentary about Rodman bringing a bunch of former NBA players to North Korea for a game against North Korea's national basketball team. Rodman was drunk the entire time. He has a serious alcohol problem. Even worse than Hillary Clinton. I'll never understand the reasoning of people who voted for a presidential candidate whose go to method of handling life's problems is to just get drunk on vodka martinis.
Rodman was so drunk that during Kim Jong-Un's birthday celebration he got up and proposed a toast in front of all the NK government leaders. Then he said that Kim Jong-Un's father "did some fucked up shit". He said that and got out of there alive.