No way it was Listach's helmet. Look at it again. The helmet falls off before Listach's head goes anywhere near Volstad. Either Listach nicked him under the pile, Morgan elbowed him while struggling or Morgan's graze caused it.
After his lunging punch, Gaby took him out, and Nyjer swung at air until he was buried and never again near Volstad. Also, after being taken out by Gaby, Nyjer was on Volstad's left, and Volstad was hit on his right - no way Nyjer did it.
EDIT: Listad's helmet does come off and doesn't appear to ever really come close to the side of Volstad's face. Volstad was the last player to get up. He could have been kicked in the face by one of his teammates.