A rabbit punch is an illegal punch where one of the boxers hits the other boxer in the back of the head. Hits to the kidneys/back are also sometimes referred to as rabbit punches.
These are very dangerous hits and it makes perfect sense that they are illegal. Not only is the victim unaware of the incoming punch (as it is occurring behind them), but think for a moment of where they are hitting - base of the head where it attaches to the spine. Wouldn't take much to snap the brain off the brain stem or cause irreparable damage to the spine.
As tempting as it may be sometimes, don't do it. The one above stopped the fight for three minutes so Hopkins could recover. Don't feel too sorry for BHop though - he's given his own share of rabbit punches in his career.