This stems from the now controversial ads that she took and paid to have in the Hollywood trade press, promoting herself.
The Huffington Post bases its conclusion on a story reported by The Daily Beast, "Melissa Leo Breaks Oscar Silence," by Jacob Bernstein, in which Leo says:
“I’ve been busting my ass, trying to get the movie sold and seen, and now I show up where they ask, get put into hair and makeup that they pay for, so I can promote this thing [and campaign]. So I’m a little confused. I thought this is what we’re doing. This is what all the girls are doing.”
The Huffington Post uses the aforementioned quote, but fails to notice THE VERY NEXT SENTENCE:
Leo adds that she conceived the ads before she was nominated—and if she had known she would wind up in contention for Best Supporting Actress, she might have done things differently. “It didn’t seem so nomination oriented,” she said. “It was fun.”
Emphasis mine. FAIL by Huffington Post.
However, note that the both the Huffington Post and the Daily Beast write that Leo was a frontrunner, and that things are not so clear anymore. Bottom line is, we'll find out on Sunday.
Yet, one comment on the Huffington Post sums up all this pretty neatly: