"Astronaut Eileen Collins is leaving NASA. Collins was the first woman to
command a space
shuttle and the leader of Discovery's Return to Flight mission last year. She
plans to pursue
private interests and spend more time with family."

http://www.spaceref.com/news/viewpr.nl.html?pid=19730

Maybe she knows something about the CEV we suspect, i.e. its going to take a lot longer than anyone realises, and also the STS is a tad too dangerous.

Sadly, the more likely reason is that her father was hit and killed by a car recently,
and her mother died last November.

The details here:

http://eileen-collins-news.newslib.com/