Because a conservative PAC prepared an and bought ad time thanking her for doing so. And, oops, forgot to pull the ad after she changed course and after the coupe didn't happen.
http://www.nbcwashington.com/news/local/TV-Ad-Thanks-Va-Rep-Comstock-Others-for-Repealing-ACA-417083513.html
So, if you were wondering how Comstock was planning to come down, before it became clear that bill would fail and Paul Ryan released her vote? Well, she apparently assured someone that she was voting Yes. And with enough certainty that they made an ad, bought airtime, released the ad to the network, and scheduled it to run. Because you don't do all that on a hunch.
I would have hated her if she had voted Yes. But intending to vote Yes, and announcing a No vote at the last minute when it became clear that the bill would fail is just slimy and two faced. At least if she had stuck with Yes, or just made not statement, she would have courage of her concision going for her. But nope. She doesn't even have that.