I am a gooner. So its possible that I see it differently.
Zabaleta got the red card first. Then Sagna. I think that usually says something, don't you? He obviously says something to Sagna which riles him up and the assistant ref hears it and that's what he tells the ref when he goes over to inquire, who sends off the aggressor first,then 'reactor' later. He sort of even wanted to explain to Sagna why he has to send him off but offered non to Zaba.
Sagna was so annoyed he kept pointing at his temple which usually implies 'brainless guy this Zabaleta'.
Obviously at first glance Sagna is the aggressor and Zaba shouldn't be sent off. But I'm convinced he said something bad to Sagna to get such a reaction. Maybe even racist. Because Sagna is usually a cool headed guy. Bowyer even stamped on him in our game before yours and got no reaction.
Someone says zaba tried to put his hand over sagna's shoulder and sagna would have none of it. Think about it. Would zidane have let materrazzi put his hand around his shoulder after he calls zidane's mom a hoe? In fact that tells me Zaba is smart enough to know he said something so bad that if he does that funny gimmick, sagna would just toss away his hand...making Zaba look the better guy!! Why would you try to put your hand around a guy who has got you sent off? (if you believe it's sagna who got Zaba sent off).
Not trying to annoy anyone here...just giving an alternative hypothesis.