Hint: It rhymes with “Yahoo.”

Arianna Huffington surprised the media world this morning when she announced that she was leaving the Huffington Post, one of the most powerful properties in digital publishing.

Huffington’s press release says she left because she had decided she couldn’t work at both HuffPo and also launch Thrive Global, her new “corporate and consumer well-being and productivity” startup.

“I thought it would be possible to build a startup and continue as editor in chief of the Huffington Post,” Huffington says in a statement. “Today, it’s clear that was an illusion.”

That’s a turnaround from a couple months ago, when she announced her plans to launch Thrive while continuing to run HuffPo, which she launched 11 years ago and sold to AOL five years ago.

A more realistic explanation, according to a person familiar with her new venture, is that things have changed at Verizon, which bought AOL last year and is now going to swallow up Yahoo.

At AOL, HuffPo was a dominant property; in a combined Yahoo/AOL, it wouldn’t have the same stature, and there will be a new set of fiefdoms and power structures.

“It was pretty clear that in Yahoo/AOL, she wouldn’t have the same position that she has now,” says the source. Huffington reported to AOL boss Tim Armstrong, and while that org structure would likely have continued on paper, in practice there “would be a lot of other people in the room.”

Couple that reality with the new money that Huffington has raised to start Thrive — $7 million in a round led by Lerer Hippeau Ventures, at a $33 million valuation — and it made the decision for Huffington easy.

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