There seem to be several derivations of BOHLER as a German name (plus a few others in other languages):
http://www.ancestry.com/search/SurnamePage.aspx?sourcecode=13304&html=b&fn=&ln=Bohler&submit.x=0&submit.y=0
Some more (German) ideas (not necessarily very likely):
1. From BALDWIN
2. Middle Low German "bole" (brother, relative, lover)
3. habitional name from "bol(l)e" (beam, board)
4. Middle High German "bühel" (hill)
Most of this is taken from:
Rosa und Volker Kohlheim, Familiennamen, Mannheim 2000