Why are Calvinists / Reformed so divisive?

Scot McKnight has published a letter from a pastor asking for help with a small and vocal group of Calvinists in his church who are being divisive over theology and not prepared to live and let live. Over 200 reader comments have confirmed that a massive number of people have had the same experiences. Scot's response thread is similar, as is Denny Burk's thread.

My own experiences and observations over the years have been clear and unambiguous - people of Calvinist / Reformed persuasions are vastly, vastly, more prone to: vocal condemnations of what they perceive as inaccurate theology, extremely harsh criticism, uncompromising insistence on adherence to their doctrine, exclusion and rejection of those failing to adhere, extreme arrogance and condescension, ungraciousness, disinterest in learning about the views of others, unwillingness to tolerate others in love or compromise, relentlessness in zeal, venomous, promoting their own views as Truth and fighting against and eradicating others.

It appears to me that this has been a growing worldwide problem for some years now. One poster describes it aptly as "a war-time mentality", which I think really is insightful, and numerous posters blame John Piper for promoting such a mentality. I doubt it's Piper alone, but it does seem to be the case that modern Calvinist apologists are prone to preaching the need to "fight" for the gospel truth and "defend" it against those who would "attack" and undermine and deny "the central truths of the faith".

What interests me is the question of why they are acting this way. Is it something in their theology which naturally leads to this sort of behavior? Perhaps. I can think of a number of possible reasons offhand:
(1) Emphasis on the importance of humans doing nothing in salvation is going to lead to less Christ-likeness in the lives of believers.
(2) Emphasis on justification by faith alone has a tendency to be interpreted as justification by belief alone, which gets interpreted as "justified by correct beliefs", and hence extreme importance is placed on correct belief.
(3) Because the emphasis is on faith alone, their Christianity is only doctrine. So often among other Christians, Christianity is about how you live and doctrine is ignored. Whereas the emphasis on faith alone cuts off the rest of Christianity and hence doctrine must be contended for because it is the center and only part of the faith in a way that it is not for other Christians.
(4) Belief in predestination is likely to lead to less evangelism, and more focus on the saved - eg teaching them correct doctrine.
(5) Their belief in the lack on any innate value in humanity is more likely to result in disdain and a lack of love toward fellow humans.
(6) The doctrine of predestination is one that a large number of people find morally and emotionally repugnant on the grounds that it is unkind and unloving. People that accept such a doctrine are thus more likely to have greater acceptance of behaviors that other Christians consider unkind and unloving.
(7) The doctrine of predestination provides an example of God arbitrarily excluding and condemning people, when imitated among humans this leads to exclusive and divisive behavior.

One poster suggests that it is "as much about a culture as it is about a theological position" and that it is the culture of harsh criticism and exclusiveness within these groups that breeds its own. I can think of a few cultural, rather than theological reasons why this might be the case:
(A) Perhaps the origin of the doctrine during the period of turmoil and persecution in the Reformation has left this sub-culture with a war mentality? I'm inclined to doubt it could have lasted that long without other more important factors.
(B) The level of propaganda / indoctrination of members among Reformed churches seems to vastly outweigh anything among other denominations. Members are trained far more heavily in their tradition than what other Christian traditions do. The subsequent zealousness with which the members defend their faith is probably greatly a result of how well they understand their own tradition and how much they see it as their own.
(C) Similarly the culture in these churches seems to place a lot of emphasis on "preaching the gospel" to believers on an extremely regular basis. Most other denominations have no interest in preaching the gospel to believers. Thus "the truth of the gospel" becomes more important to these churches because they hear it often.
(D) Up until 30 years ago the vast majority of conservative biblical scholarship supported Calvinism, but in the last 30 years conservative biblical scholarship has vastly improved for a variety of reasons and as a result has systematically undermined, demolished and disproved the exegesis on which Calvinist ideas were based. Calvinist apologists have responded by attempting to fight scholarly biblical exegesis which has had a trickle down effect and infected their followers to "fight" for their gospel.
(E) It's a recent fad, it will pass like all fads do. Perhaps certain recent writers have contributed by writing works in an inflammatory fashion.
(F) The introduction of the internet has affected Calvinism in a way it hasn't affected other Christian denominations so much (probably because Calvinism focuses more on ideas rather than practice, which are thus more easily discussed).

So those are a range of possible factors that I can come up with. I'm left wondering if I've thought of the right factors, and if so, which of the factors I've thought of are the most important ones. My best guess at what I see as the most likely explanation is: Calvinism has lost massive ground in biblical studies in 30 years with the majority of conservative biblical scholars now agreeing that fundamental doctrines of creedal Calvinism were built on biblical misinterpretation, which has led popular Calvinist Apologists and Scholars to launch a defensive attack to "fight" for their gospel and so they see themselves as being on a war-footing which as a mindset has filtered down to their followers, and done so much more than it once would have as a result of the internet, and the situation has been worsened by a number of theological and cultural factors which cause Calvinists to be more than usually prone to this sort of behavior.