The two of them came at him at the same time and sandwiched him, similar to the hit that knocked Ben Roethlisberger out of the game. Hali was flagged on the play for unnecessary roughness, he "struck the opponent with his helmet", according a league spokesman.
This is the hit and, yeah, I think Tamba did break the rules. Tamba's the one opposite Justin Houston (50).
It looks like he launched himself into Leftwich. Not saying whether I think it's a dirty hit but I think it did violate the rules as they're written, the way he led with his helmet.
[Update: Well, AP user Knightwolfbr makes a good point here I missed: "Didn't Hali jump in the air to try and knock the ball down? Could that have anything to do with it?"]
The fine is for $25,000.
That penalty hurt, too. The Steelers were on facing a 3rd and long at the 27-yard line. A field goal from that distance isn't the gimme that one from the 13-yard line is, which they nailed.
In other fine news, the NFL does not like Justin Houston's dance moves. The league fined him $7,875 for showing off his dance moves.