Both Mayim Bialik and Ken Jennings take turns hosting Jeopardy, and both tend to make headlines for the job that they do on the show - Ken over the jokes he makes, and Mayim for just about everything she does. Jennings recently handed the reins back over to Bialik, however the Call Me Kat actress has abruptly left her position behind the podium a week early, so producers had to bring Ken back in to finish her shows.
As to why Mayim walked off, it is in support of the ongoing writers' strike that started last week. The strike has seen Writers Guild writers from many different TV shows hit the picket lines for fairer wages. Jeopardy uses Writers Guild writers, and they have walked off the job too, with one telling Variety, "Our words are on the screen every night. There is no Jeopardy without writers. Without us, it's just an empty blue screen."
Ken will finish out the last week of shows for Jeopardy's 39th season, filming them next week.
Jeopardy is the first game show to be affected by the writers' strike.