What I saw: Varitek's extra-base hit rattled around the right field corner. Instead of trying to cut it off, Sheffield waited for it to roll along the wall to him. One fan in a red jacket was reaching down for the ball and another fan (in a blue jacket) was trying to pushing him back from the play.
I assume Blue Jacket knew that if Red Jacket touched the ball, it would be a ground rule double, but if he didn't, Varitek had a shot at a triple. So as Blue Jacket was reaching down and over to push Red Jacket out of the way, he may have brushed Sheffield, who was just about to field the ball.
Sheffield got the ball, but before throwing it back to the infield, he turned and swung both hands at Blue Jacket. After getting rid of the ball -- Varitek did get a triple -- Sheffield whirled around again, ready to fight. Fortunately, all of the fans along the wall had backed off, Sheffield remained on his side of the stands and a security guy raced over from the bullpen to keep order. The fan was ejected from the park, but I don't think he was arrested.
Sheffield: "I just felt something hit me in the mouth. I don't know if he hit me or not, it felt like it. I thought my lip was busted. ... To get punched in the mouth, you don't expect that in a baseball game."
Except you didn't say you were punched. In fact, you're not even sure you were hit. ... I'm sure the fans were giving it to Sheffield all night, which doesn't help matters, but I've yet to see a replay that showed any intentional contact between a fan and Sheffield, let alone an actual punch. Photos.