I guess Bucky was perhaps not happy about being passed over for the interim job. Or Mackanin wants his own guy:

From DDN.com

Dent out as Reds bench coach
CINCINNATI -- There will be at least one coach replaced when new Cincinnati Reds manager Pete Mackanin takes over today against the San Francisco Giants.
Pat Kelly, manager of the team's Gulf Coast Rookie League Reds, will replace Bucky Dent as bench coach.
Kelly spent last season as a professional scout for the Atlanta Braves and returned to the Reds for this season. Prior to that, Kelly managed Chattanooga, the Reds' Class AA affiliate in the Southern League, in 1993-94.
Kelly, 51, has managed 12 different minor-league teams during his career after retiring as from active play, mostly as a catcher in the minor leagues, making the majors with Toronto in 1980.