What version of backburner are you running, I have had issues with the new 2012 backburner, so if you run with 2012 try installing backburner 2008.1.

Another thing could be that you have a SLM version of the software which don't have Mental ray batchnodes( only the NLM or network license has them), which means you are rendering with satelite and satelite only takes the 1st core of a dual core machine hence the 50 percent....

And at the end a little tip...don't use Backburner, it's rubbish, try Smedge or Royal render instead:
Smedge: https://www.uberware.net/download.php
RoyalRender: https://www.royalrender.de/cms/downloads/

You have to pay money for them but setting them up is a piece of cake whereas BackBurner is a pain and only if you use a flame or smoke your stuck with it , most postproduction facilities I know off have teh aforementionde Smedge or Royalrender as their render manager...