YIKES!!!
Did you use a 1/4-20 nut or an M6 nut? Make sure that you're using an M6 nut. You've obviously got the positive terminal connected fine, though. Is it possible that you've got an M6 on the positive and a 1/4-20 on the negative?
http://www.we-online.com/website/fra...rDOCID=1010821
The PD uses the Wurth 7461098 shank for connections (see above for datasheet in "power elements"). Unfortunately, the above link doesn't include such specs but I'm guessing that someone knows how much torque is required to snap an M6 bolt of ? metal. I guess that it's possible that your shank was somehow defective in its metallurgy or machining.
At work, we call this much torque a "Hillis" or a "mega Hurley" (also 1 microHillis = 1 Hurley) in honor of a few of our staff. You might want to try using a shorter wrench next time (or maybe finger-tight in your case)
Oh snap...
Russ