He basically sat on Meb's shoulder the whole time and refused to take the lead at any point, despite Meb basically saying to him during the race that he wanted Galen to stop riding right on top of him. They had pulled ahead of most of the field and I think Meb's perspective was, why not work together some to put these guys away altogether. Instead, Rupp just sat back and executed his race plan. I can get why Rupp took that approach - this is an each man for himself sport - but it still seems like a douche move.
I am almost certain Rupp was doing what he was told to do. What would be the purpose of Rupp working with Meb when it was pretty obviously they were going go 1-2? When they separated, the only thing Rupp wanted to do was win, which he did pretty easily.