Check this:
http://www.ancestry.com/search/SurnamePage.aspx?sourcecode=13304&html=b&fn=&ln=Reder&submit.x=0&submit.y=0
In addition to this my dictionary of German surnames (Rosa und Volker Kohlheim, Familiennamen, Mannheim 2000) has:
- occupational name, from Middle High German "reder" (miller's servant)
- from "reden" (talk): someone who talks a lot, laywer
- same as Rader (maker of wheels)
So take your pick