5:45 PM PT -- A source close to Conor tells us the fighter wanted to say hello to MGK, stuck his hand out and started walking up to the singer. We're told MGK said something -- that Conor couldn't hear -- and MGK's security pushed Conor away. Our source says Conor doesn't understand what the issue was, and was surprised by how he was treated.
5:31 PM PT -- Sources connected to the situation tell us Conor asked MGK for a photo, which Kelly denied ... and that apparently escalated into him pushing Conor, whom we're told stumbled back and spilled his drink.
When he collected himself and whatever was left of his drink, our sources say Conor allegedly chucked the drink at Kelly ... as well as Megan, whom we're told was nearby.
Their respective teams broke it up before things got really ugly -- and Conor was given his walking cane back -- something that had fallen to the ground during the scuffle. FYI ... Conor made it inside after this -- the camera just panned over him during the broadcast.
Conor McGregor is still nursing his broken leg, but dude is already back to squaring up -- only this time ... he's doing it well outside the Octagon, and against Machine Gun Kelly.
The UFC star appears to have gotten into a physical confrontation Sunday on the red carpet of the MTV Video Music Awards leading into the Barclays Center -- right before showtime, no less -- in a would-be clash that was captured by several photogs on the scene.
As you can see, there's Conor in the pink suit mouthing off to MGK who's not very far ... and before you know it -- the next shot has Conor seemingly lunging at the rocker as people step in to separate them.
CMG can be seen reaching for Kelly -- but it doesn't look like he ever got a hold of him or even laid a finger on him for that matter. Conor was yanked back by security on hand ... with the Notorious looking furious and animated amid the scrum.
Unclear what might've set this whole thing off in the first place -- but it doesn't seem this escalated beyond a brief showdown ... because we've reached out to cops in the area, and no one's reported this (yet). It's weird ... MGK and Conor don't really have a hostile history.
We got calls in to both guys' camps to see what the fuss was about ... so far, no word back.