I suppose it's just human nature. If I deviate from your accepted standards and norms too much, you shun me. At its extreme, this is the source of racism and even genocide.
There is a continuum of disfellowshipping: from burning someone at the stake, to formally shunning them, to avoiding social interaction with them, to mentally thinking there is something wrong with them — that they are not to be trusted.
Christianity has various divergent views which commonly result in disfellowshipping. For example, some well-known and influential Calvinists strongly teach that Christians who reject Calvinism are not saved. Does calling another Christian unsaved qualify as disfellowshipping?
I long ago rejected young-earth creationism and was disfellowshipped for it. There is heated division between the two camps. For some young-earth creationists, it's an essential doctrine of the Christian faith.
I long ago rejected the charismatic movement with its speaking in tongues and wild manifestations of the Holy Spirit and miraculous healings. The word-faith health-wealth movement naturally arises from taking charismatic beliefs and practices to their logical conclusions.
I long ago rejected premillennialism and the rapture; this, in favor of amillennialism: that there was no 1,000 year millennium at all. I don't think it's an exaggeration to say that I was mildly disfellowshipped for this. Certainly, among those who speak of such topics often, I was left out of the conversation completely.
And there is a radical disfellowshipping among Catholics and Orthodox and Protestants, even among Protestants of various types — evangelical fundamentalists, "denominationalists", liberal, and etc.
I find it odd that Christians so easily disfellowship one another. Aren't they supposed to be spiritually more enlightened than others? Or does brute human nature inevitably dominate everything else?