Fans are concerned for his safety, and are hoping he is not going through any personal problems. It doesn't appear as though he has quit or has been fired, as some mention of that would have been made.
There is a potential contract dispute between Sportsnet and Marek that could potentially explain his departure, as we have seen many other analysts and broadcasters in the past stop working due to this.
Fans sure hope Marek will be back shortly, or that an announcement will be made, as his show with Friedman has a huge wave of supporters and great on-air ratings. For the first time ever, Amazon will be broadcasting NHL games next season, so perhaps they are trying to drag Marek along with them. Only time will tell.