This is the first time I've watched the whole clip as it was broadcast (the taunting, not the bumping the ref part).
Marsh fucked up, and based on what the ref saw he was correct to throw the flag. He was wrong, mind you, but from his perspective Marsh was taunting the punter. The ref misinterpreting what he was seeing is unfortunate, but the truth is that Marsh was deliberately directing his antics at the Steelers bench, he certainly wasn't trying to hide it, and he wound up interacting with the punter whether it was his intention or not. Kev's speeding analogy really takes a new light here. He created a situation where his actions were misinterpreted.
Watch where Marsh, the punter, and the ref are in relation to each other. After Marsh is already shouting at the steelers bench, how does the punter interaction look from the ref's vantage point? What really got him was the sidestepping in the path of the punter, I reckon.
Ironically, the reason he took such a direct path at the ref is because he'd been staying in front of the punter, not taunting him.
edit: Also, I didn't realize that this was the same guy that didn't have any fun playing for Bill and the Patriots. Hope he enjoyed himself Monday night.