In another chapter to the growing rivalry between the two clubs, Cox ran into the Giants' warm-up area during a ball-up drill before Saturday night's opening-round match at Engie Stadium.
The Magpies premiership ruckman shoved an unaware Mumford from behind, before jumping up and running away with the ball.
Collingwood and Cox were asked to explain the incident to the AFL.
"I apologise for the incident in our pre-game where I made contact with a GWS official," Cox said in a statement on Monday."In the moment, I was solely focused on getting one more bounce in - in hindsight, I should have stopped my run-up rather than completing the drill."
Collingwood said players and officials from the two clubs were crossing over as they undertook their centre-bounce drills before the preliminary final rematch.
The Magpies insist Cox asked an umpire to bounce the ball one more time to finish his preparation.
"We have reviewed the situation and it is clear it was a period of transition in the centre circle - our rucks were completing their centre bounce practice as the GWS midfield arrived to commence their centre-circle work," interim Collingwood football boss Brendon Bolton said."Mason has acknowledged that making contact with opposition officials or players pre-game should be avoided and with our club's support, will be more aware in future."
The AFL are still investigating the incident.
Giants' ruck coach Shane Mumford was the unwitting target of Cox's push. (Richard Wainwright/AAP PHOTOS)
Giants forward Brent Daniels didn't know about the episode until watching a replay of the Giants' 32-point win, but took full aim at Cox.
"Mason likes to say things that get a bit of a headline," Daniels told the Nine Network on Sunday.
"He likes the limelight, but what he did before the game was really bizarre. A bit of a carry on.
"I'd like to think we're really humble and hungry as a team.
"To see that wasn't a good look."
Cox was already public enemy No.1 with the Giants after describing their home venue as "a showground for livestock" when the AFL announced it would be starting the 2024 season with four games in NSW and Queensland.
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