I don't know what was behind her late endorsement of O'Donnell. I know she had a chance to meet and talk with CO at the Beck rally, so she had some sense of Christine from a face-to-face meeting.
My guess is that with her recent endorsement successes she felt she could afford this "swing for the fences." Believe me, if O'Donnell wins it will be a huge victory for grassroots conservatives. The Beltway GOP will be rocked.
I think you've pegged it exactly. There is a time for cautious political calculation and there is a time to trust your instincts. Her instincts are pretty good... /g