Since I don't experience this sort of urge (to shun), I find it difficult to understand the logic behind it. I've been looking into the exact definition of Shunning and the psychological cause & effect. Much to my surprise, Shunning is considered torture. (Aren't you all proud of yourselves?)
The following information has been taken from good ole' Wikipedia. (I will dutifully supply a link to the page at the bottom of this post.)
Shunning can be the act of social rejection, or emotional distance. In a religious context, shunning is a formal decision by a denomination or a congregation to cease interaction with an individual or a group, and follows a particular set of rules. It differs from, but may be associated with, excommunication.
Social rejection was and is a punishment used by many customary legal systems. Such sanctions include the ostracism of ancient Athens and the still-used kasepekang in Balinese society.
Shunning is often used as a pejorative term to describe any organizationally mandated disassociation, and has acquired a connotation of abuse and relational aggression. This is due to the sometimes extreme damage caused by its disruption to normal relationships between individuals, such as friendships and family relations. Disruption of established relationships certainly causes pain, which is at least an unintended consequence of the practices described here, though it may also in many cases be an intended, coercive consequence. This pain, especially when seen as unjustly inflicted, can have secondary general psychological effects on self-worth and self-confidence, trust and trustworthiness, and can, as with other types of trauma, impair psychological function.
Shunning often involves implicit or explicit shame for a member who commits acts seen as wrong by the group or its leadership. Such shame may not be psychologically damaging if the membership is voluntary and the rules of behavior were clear before the person joined. However, if the rules are arbitrary, if the group membership is seen as essential for personal security, safety, or health, or if the application of the rules is inconsistent, such shame can be highly destructive. This can be especially damaging if perceptions are attacked or controlled, or various tools of psychological pressure applied.
Extremes of this cross over the line into psychological torture and can be permanently scarring.
A key detrimental effect of some of the practices associated with shunning relate to their effect on relationships, especially family relationships. At its extremes, the practices may destroy marriages, break up families, and separate children and their parents. The effect of shunning can be very dramatic or even devastating on the shunned, as it can damage or destroy the shunned member's closest familial, spousal, social, emotional, and economic bonds.
Shunning contains aspects of what is known as relational aggression in psychological literature. When used by church members and member-spouse parents against excommunicant parents it contains elements of what psychologists call parental alienation. Extreme shunning may cause traumas to the shunned (and to their dependents) similar to what is studied in the psychology of torture.
Shunning is also a mechanism in family estrangement. When an adult child, sibling, or parent physically and/or emotionally cuts himself off from the family without adequate justification, the act traumatizes the family. 
This is probably the last time I will post on this topic, as life is moving on, and I'm moving right along with it. However, I would like to acknowledge an interesting point, that the one thing I find connected to individuals who practice Shunning, in all of the above scenarios (both family & community), is extreme narcissism, jealousy, and a need for control.
The observation of and research into people who "Shun" could be a psychological study in and of itself. (And it just might be somewhere, who knows.)