Yesterday there were runners on second and third with no outs. The batter hit a hard ground ball that struck the field umpire in the leg. Both runners advanced and the batter reached first base safely. The home plate umpire then came out and said "if the ball hits an umpire behind the pitchers mound, the ball is dead and runners have to go back to their previous base." I have never heard this rule and it makes zero sense to me, any explanations? Thanks!