Seems I remember reading the actual story on this either in another thread or on another site... if memory serves, the umpire charged with the assault was not the umpire involved in the argument initially, and during the argument between a different umpire and coach, this umpire grabbed the coach by the neck and threw him down... and then proceeded to get his *** kicked.
I confess that I may not be remembering perfectly, but I do remember when reading the story the first time if the umpire would be charged, as it sounded like he was the instigator.