Denny, the brush holders are not on exact center. when the housing is rotated 180 degrees, a different set of commutator bars is under the brushes and it sets up a different magnetic field. That causes the armature to rotate in the wrong direction when its field reacts with the one created by running current through the field coils. It's all a matter of geometry between the two sets of magnetic fields.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!