All or nearly all atheists who have discussed religion with Christians or who have simply been approached by Christians have probably had the surreal experience of being sagely informed about what they believe, how they believe, and even why they believe. Christians may lay claim to a variety of supernatural powers (like healing prayer), but when did they acquire the ability to read the minds of people they don't know and have never met? Does Christianity really provide mind-reading powers?
Of course not. Christians have no more ability to read atheists' minds than they do to heal through prayer or faith, but the fact remains that far too many act like they can read minds, so what's going on? I suppose there may be a variety of factors, depending on the individual, but one common feature seems to be that they have bought into a collection of myths about atheism and atheists.
Instead of initiating a conversation with another human being by asking them what they think and why, some Christians simply assume that they already know whatever it is they need to know and proceed from there. This might be legitimate — at least up to a point — when dealing with adherents of a clearly defined ideology. If you were talking to a Objectivist, for example, it would probably be fair to assume that they accept a number of basic Objectivist principles or ideas. When it comes to such broad categories like atheism and theism, however, there's hardly anything that can be assumed.
This can be a problem which some atheists have with theism as well. In the context of America the label "theist" only rarely refers to someone who isn't Christian or who doesn't at least believe in some version of the traditional god of Western philosophy and theology. This can permit some initial (though provisional) generalizations in a conversation with a "theist," but it's still smarter and politer to simply ask what a theist believes. You'll do this a thousand times and get a thousand repetitions of a standard Christian creed, but number 1001 might still be different.
Making lots of broad assumptions about who atheists are and what they believe allows a Christian to comfortably relegate atheists to a pre-defined and pre-dismissed category. There is no need to inquire about their actual beliefs if their actual beliefs don't matter, and their actual beliefs won't matter if they are simply playing a pre-defined role in someone's religious ideology.
Though not as obvious as when people go so far as to demonize another group, this sort of pre-categorization is still a form of dehumanization. It effectively undermines the common insistence that they only want to "understand the atheist's point of view." In truth, it's more likely that they just want validation for what they have been taught and whatever you say will go through a perceptual filter, changing what you say until it looks like what they were told to expect from atheists.
I'm frequently amazed at how things I say get twisted around into something completely unrecognizable. Everything about me, from what I say to my very existence, stops being about me and becomes part of their ideology. I'm no longer an independent human being with my own life and my own perspective; instead, I become a bit character in a cosmic play about Good and Evil. It's not even a very good part because there's no room for backstory or my own story line. I'm just their to prop up the main characters' roles.
One common feature in all of this is arrogance. It's arrogant to presume to know what a person thinks simply because they are an atheist, never mind the manner in which they hold their beliefs or why they believe what they believe. It's arrogant to presume to know all you think you need to know about atheism and atheists based on little or nothing more than the musing of a few Christian apologists. It's arrogant to claim to be interested in people as individuals but not care enough to actually ask them what they believe. It's arrogant to treat a person and their beliefs as little more than props in the drama of one's own religion.
Notice how frequently atheists are the ones accused of being "arrogant" for nothing more than daring to dismiss popular religion and, what's worse, publicly declaring the reasons for this rejection. Atheists are supposed to be arrogant because they don't privilege popular religion, treating it like adherents of popular religion tend to treat minority religions or belief in Bigfoot, while Christians aren't considered arrogant at all for these common attitudes towards atheists and atheism.
This is just one more in a long line of situations which demonstrate that accusations of arrogance against atheists have nothing to do with atheists actual behavior, but rather are part of a larger agenda to silence atheists. It's not that atheists are arrogant, but that they refuse to be cowed by the genuine arrogance of religious theists who demand their beliefs and their ideology be privileged. Standing up to arrogance is no more arrogant than standing up to bigotry is bigoted.
Unfortunately, Christians' arrogance and privileges have gone for so long without challenge that atheists' challenges today are taken very personally. This is only to be expected because the challenges to other forms of unjust privilege — male, heterosexual, white, etc. — have been met with similar reactions and similar accusations of "arrogance" against minorities. We can't avoid the accusations but we can't avoid imitating their behavior and stick to our guns when challenging religion in society.