My understanding is that Jackson Oneil punched the ball forward (away from the goals) and a United player then punched/kicked the ball back through for a behind. If you read the rules this was to United's advantage and should be called a no score. Wills was the umpire who made the call. Was icing on the cake for a terrible display of umpiring.
law 12.5.2 Football touched in Transit
A Behind shall still be recorded under Law 12.5.1 if
the football is touched in transit by another Player,
provided the field Umpire is satisfied that the
scoring of the Behind was not assisted by a Player
from the same Team.