Hi, I cannot pretend that I am the best person to answer this- I'm mainly waiting, like you, for an answer to your excellent query.
I can think of a few reasons, though,don't know if these are as specific as you would like...
- anger problems or psychosis in the abuser - mild to severe sadism in the abuser - jealousy in the abuser (as in cases of termination of romantic relationships) - vengeance (as 'payment' for a real or perceived wrong done by the abused to the abuser)