Why does the image in my reflector telescope appear slanted when viewing?

It depends on the angle of the eyepiece holder relative to the horizon. If the eyepiece holder is vertical to the horizon, and you view with your back to the object, the image in the eyepiece will be erect and not slanted.

Further to GeoffG's answer...

If you have a Newtonian Reflector then the image should simply be upside down. If, on the other hand, you have a Schmidt-Cassegrain, then the angle of the star diagonal relative to the horizon also comes into play. If you were to orient the star diagonal such that the eyepiece were vertical, then the image would appear upright.

You could rotate your body about the eyepiece the appropriate number of degrees to view the horizon at the angle of desire. Not sure why this is important. Astronomers don't really care how the image is oriented in their eyepiece, aside from using star-hopping techniques, in which case they make the adjustments in their head to move to a desired target.