He didn’t really “run up to” Seymour. There were a bunch of Steelers in the area and Seymour, and they were yacking and celebrating and Ben was moving forward, towards more Steelers, and Seymour hit him. Not really sure what Ben could have said in that 2 seconds that made Seymour that mad, but it didn’t go on long enough to warrant ejection. Looked like only two or three syllables tops, to get ejected for talking you’ll have to really push it deep into gross and probably racial territory, I don’t think Ben had long enough to go that far.
You are exactly right. In fact, I don’t think that Ben said anything to Seymour. It looked to me like he was looking past Seymour talking or shouting to someone forward of Seymour. There were a bunch of bodies there and it looks to me like Ben was walking towards someone (possibly another lineman??) saying something to that person, and it looks like he puts his hand on Seymour’s shoulder pads as if to say I’m coming by and then Seymour turns around and smacks him. Seymour has a history of angry outbursts so it’s no surprise he does this.