2. Winner's Nam Tae Hyun leave due to mental health issue.
The agency explained that they were aware of Nam Taehyun’s mental health issue since his trainee days, but his condition had come into attention when it had worsened during the past few months. Therefore, YG Entertainment announced that plans for WINNER to release a new song have been postponed indefinitely. “After discerning that it would be difficult for him to continue promoting, we have decided that his future activities will be suspended in order for him to focus on treatment. This has been done because the most important priority is his health. He has been staying at home with his mother and is receiving treatment instead of staying at the dorms,” explained YG [AsianJunkie]

But then Nam Tae Hyun officially leave WINNER (november 2016)

5. Jung Il Woo suffered cerebral aneurysm
Jung Il Woo, a popular Korean actor, is well known for his role in The Night Watchman’s Journal (2014) and Cinderella and Four Knights (2016). Having made his presence felt in theater, TV, and films, the actor holds a wide reach in the entertainment industry.

The thing about South Korean national policy is that all male citizens between the age of 18 and 35 need to undertake mandatory military service or compulsory national service. As a part of the policy, even entertainment stars are enlisted for the same. Jung Il Woo and Lee Min Ho, however, were not able to enlist into the service because of an accident.

In 2006, two big stars, Jung Il Woo and Lee Min Ho, met with a serious car accident. The crash led to Jung having a shattered wrist bone, fractured pelvic bone, and internal head hemorrhage. Lee Min Ho also suffered a broken right thigh and ankle bone.

Due to the accident, Jung Il Woo could not take up the enlistment. However, the actor said that he is disappointed that he cannot serve his country in the fullest way. [Source]

6 Song Ji Hyo and Kim Joong Kook are officially leaving 'Running Man'.
On December 13, insiders revealed that a new season is in the discussion for next year with a new member, Kang Ho Dong, joining, and current members, Song Ji Hyo and Kim Jong Kook, leaving the show.
Song Ji Hyo's label MY Company confirmed the reports, and announced, "Song Ji Hyo is leaving 'Running Man'. There was a long discussion with the producers as it was overwhelming for her to handle both acting promotions and a variety show. She plans to focus on her acting career after leaving 'Running Man'. The date of her departure is currently in adjustment."
Kim Jong Kook's label Maroo Entertainment also announced, "Kim Jong Kook decided to leave 'Running Man'. The exact date of departure is not yet confirmed."
As a result, Song Ji Hyo and Kim Jong Kook are officially leaving 'Running Man' following Gary. [AllKpop]

Kim Jong Kook and Song Ji Hyo's labels confirm they were blind-sided by 'Running Man' producers
Following news of Kim Jong Kook and Song Ji Hyo's official leave from 'Running Man', both stars' agencies have confirmed they were blind-sided by the hit variety show's producers.
Though the actress' agency MY Company and the singer's label Maroo Entertainment did not allude to the fact that they were essentially forced out of the show in previous reports, they've now confirmed neither were given a proper notice. On December 14, reports revealed, "Two weeks ago, there were talks of a format change, and a co-producer visited the filming set to announce it again. There were no talks of member changes at the time. On December 12, they had a meeting with Kim Jong Kook to tell him he was off the show. Song Ji Hyo only found out after the news made headlines. Everyone did know a change would happen as the ratings were dropping, but this type of farewell not only hurts the producers but the featured stars as well."
Song Ji Hyo's agency stated, "We knew 'Running Man' would undergo format changes from SBS, but it's true we found out from news reports which members would not return. We won't comment on Song Ji Hyo's feelings on the matter." Kim Jong Kook's label also commented, "It's true he was told he was leaving with no decision about the matter. It's disappointing. He featured in the show despite his health issues because he cared about it so much."
On the new reports, producers of  SBS' 'Running Man' said, "We can't confirm anything related to their leave." [AllKpop]

**Rumour said that Kang Ho Dong will join running man, and the New Ace is Kim Tae Hee. Also the producer of running man give some hints that Song Jong Ki may join Running Man again. 

In my opinion, it's possible to let Kang Ho Dong and Kim Tae Hee joining Running Man Season 2. But Song Jong Ki, I think he won't join Running Man, even there are Lee Kwang Soo, but Joong Kook and Ji Hyo are like hyeong and his noona. Thinking it's Song Jong Ki, he won't accept main cast Running Man because of what producer did to his hyeong and noona. 

I have a feeling, Lee Kwang Soo will leave soon, because his schedule may be so crowded by next year.

 Also before this news blown up to social media,I read SBS Running Man instagram announcement for asking people who already participate in Running Man to send email to RM staff latest on 14 Dec 2016 (unfortunately I didn't caps it orz) I already told my friend via Instagram when they posted those announcement, I said to her, 'They were up for something' and just a couple of days this news appear to mass media. I am a bit disappointed with SBS Running Man producer, they kicked main cast just because of the incapability PD. I think it's not a wise decision, I don't know, if they will held proper farewell party for Song Ji Hyo and Kim Joong Kook or not. And I know Korean Entertainment industry is so cruel, variety show will be easily canceled if they didn't get high rating.