This was just announced a few minutes ago, so very little is known about the reasons for Current's decision. My theory is that the channel wants to be more centrist Democrat, so I'm thinking that Cenk Uygur of THE YOUNG TURKS--someone not afraid to disagree with President Obama--may be vulnerable to pressures from above.
In Keith Olbermann's case, he allegedly tends to fight Authority until Authority shows him the door.
[UPDATE] Via THE NEW YORK TIMES, Current TV's side of the story:
Here's a post from the entertainment site THE WRAP:
[UPDATE] Olbermann, via Twitter, tells his side of the story: