For those that didn't realize, AC called it a little earlier in the 4th (or it might've been the first OT). LeBron had been barking on a few plays about the second defender defending the pick player, not switching onto George (must be the assignment). But George was consistently left open on the same screen play because there was no help defense.
Then in that 2nd OT, Tristan backed off him and defended coverages and space as opposed to the man and George hit that 3. LeBron then verbally murdered him and Tristan yapped right back at him. They were still going as they went to the huddle for the TO - that's when Lue tried to intervene and to get them both to relax.
I highly doubt that this had anything to do with Lue and his rapport with the players as some have suggested. This was two guys going at it, calling each other out, while the coach did his job in terms of trying to mitigate the situation.