Could be some political rivalries at play here too. Many people, at least several months ago, viewed Bachmann as a Palin wannabe. Perhaps Bachmann sees an opportunity here. If she thinks Sarah has backed a losing horse, perhaps she thinks siding with a potential winner (Romney) will boost her political stature at the expense of Sarah's.
Bachmann is a poor man version of Palin. What stature she had, she lost running for President, she harmed herself more by actually running for President than not by her obsessive repeating the same lines over and over again (23 foster kids, I’m a tax lawyer), her Gardisol flap and not really going after Romney—something Palin wouldn’t have done. Even today if Palin is on a show, the ratings go up a bit, I doubt many people across the nation are waiting to hear what Bachmann has to say much.
Good point.