After he endorsed Romney. Very strange and confusing for the MSM. Here’s the backstory:

The Iowa congressman kept his endorsement choice under wraps so closely that no one knew who he would choose between Thompson or Romney for his nod. The vociferous critic of illegal immigration was said to have narrowed his choice to the two candidates, but he joked that “not even his wife” knew who he’d choose.

But at the start of a sparsely attended press conference in Des Moines today, the superstars of Romney’s Iowa shop gathered in the back, leading journalists to conclude that King’s nod was destined for the Massachusetts governor. But then curious members of Team Thompson swooped in at the last minute.
[…]
As he detoured into a treatise on religious tolerance — a clear reference to Romney’s Mormonism — a Romney endorsement seemed imminent. “I think it’s inappropriate to draw conclusions about what we think someone else’s faith might be,” he said.
“There’s only one candidate who epitomizes the full spectrum of our conservative values,” he continued. “There is one who is a comfortable conservative, in whom I have full confidence in his decisions.”
“He also has the fire in his belly to go the distance, and the brains and resources to get there,” said King.
Nail in the coffin for Thompson, everyone thought in the room. (Thompson is frequently caricatured as sluggish and apathetic about his White House hopes, and his campaign coffers are paltry in comparison to Romney’s millions.)

Then King added, “If it’s not there, I intend to work hard to put it there.” The typing and the scratching of pens in the room suddenly stopped as reporters’ heads snapped up. “I will put all of my efforts — and the fire into the belly — of the person who I believe destiny has called to be the president of the United States.”
“I will be working for and supporting Fred Thompson for president.”
[…]
And more than one reporter re-learned a hard lesson … it ain’t over til it’s over.

Yes, it seems a lot of people should learn that lesson.