starlight,
If you have a free shot at someone from behind, you don't need anything other than your should to inflict a devastating hit. If Rand Paul's arms were away from his side, even slightly (as would be if his hands were on the handle of the lawn mower) ... and you had a few steps to build up a little speed ... lowering your shoulder into the rib area can easily cause such injuries.
If his neighbor had used a weapon of some sort ... such as a baseball bat, etc. ... he would be in jail without bond for assault with a deadly weapon, right now.
B.