It's pretty obvious it was a control room visual cue.
But the very fact that speculation of a darker nature has had such legs is testament to the state of the American media and the public's opinion of it. The fact that it was so widely believed to be deliberate on the part of some anti-Bush agenda at work should serve as a wake-up call to CNN and the rest of the media. The fact that CNN was forced to defend against the suspicion demonstrates just how much they have lost credibility with the American public.
"Kill him! But make it look like an accident." Plausable deniability. "It was an accident! Really!"