If it was a real fragmentation grenade, the shrapnel would have killed the guy that tossed it off the field... at least maimed him. If it was a grenade, it might have been a practice grenade... which is pretty explosive in it's own right.
My brother-in-law told a story about coming back from National Guard training with a practice grenade. The story starts out with a Vietnamese family that moved in the week before. Their driveway was directly across from my brother-in-law's.
Anyway, my brother-in-law was showing his roommate the grenade. He then threw a practice grenade pretty hard down his driveway and it rolled across the street, up the other drive and onto the Vietnamese people's carport, where it blew up really loud.
My brother-in-law and his roommate quickly ran back inside their house, half laughing, half thinking "Oh shit!"
The Vietnamese people moved out the next week.