I get that. I don't know if you've ever played football at any level, let alone the NFL. When you are tackling someone, you don't hear a lot let alone hear clearly. At the same time, there were only three people in a position to hear (Garrett, Rudolf and the Steelers OLmen, and the steelers OL was several yards away when the tussle began). All of them were engaged in a very loud environment.
So it doesn't surprise me one bit nobody else heard it. It really comes down to the QBs word against Garretts. And, as I said, Garrett may have well thought he heard something that Rudolf didn't say. In the video, you can see it was a clean play until Rudolf starts in with extra curricular activities. Both are on the ground a couple of yards from anyone else. Rudolf starts pushing and shoving. The Steelers OL pulls the two of them up and separated them, then Rudolf comes after Garrett. Garrett then swings with Rudolf's helmet.
Finally, Garrett has no history of this before or after this incident. So what got him so upset? I don't know.