Because most Xbox owners are American and therefore more prone to shooting each other and/or defending their right to do so.
While that sounds like a joke, it could be partly true. ( the part about most xbox gamers are US folks)
On US xbox vs playstation war is kind of balanced, which shows up on forums/internet when US xbox gamers talk about next gen and they are clueless how brutally more popular playstation is outside of US, like it is not even a competition between them. So in their perspective anything minor can turn the scale and make xbox "winner", while from outside perspective it is just illogical as playstation would have to do _everything_ wrong and they would still have a chance to "win".
That alone have given me the image of typical xbox fan being kind of aggressive and not too informed about global situation, when looking all these threads I rarely have seen PS-fanboys being so stubborn/angry/mean to others or bashing other products so aggressively.
And we all (at least outsiders) know how different US culture is vs. hundreds of other cultures, mainly their patriotism, violence, gun culture, lack of caring about poor people (there are no homeless people in many countries as everyone gets an apartment rent paid if they want to).
But difficult to say how xbox gamers are outside of US, as I dont know anyone whom is "xbox gamer", that is how popular that brand is here.
Most aggressive gamers I know are NHL / FIFA gamers + COD etc shooters. Those idiots scream in the middle of the night when they lose, throw their controllers into a wall, rape their controller in rage instead of using it normally (gently but precise).
Maybe they should study them.
But if this study is made from US xbox vs US playstation gamers, then it is more interesting thing as cultural variables are removed from the equation