For me, a lot of the disdain in my tone and voice (when it is there) comes from years of wasted time, and I'd be lying if I didn't feel like I had been betrayed. I can't pinpoint the source of that betrayal, but it's likely if anyone is to blame it is me. I blindly believed what I was told because I had no choice. The dogma was forced upon me during my formative years and I failed to investigate further the moment I had doubts. Instead, I just ignored it.
Once a person leaves a religion, especially one that makes you fear a made-up dictator, you tend to feel a tremendous weight off your shoulders, but you also can't help but realize there were years of wasted time. For me, I can never get that time back obviously; I can never have those guilty feelings remedied, and only now can that fear subside, but that still doesn't help that young boy who was told he'd eternally burn in hell for taking a pack of baseball cards from the local convenience store.
This can explain why nonbelievers seem strident and angry, because they not only resent these aforementioned reasons, but they also don't want others to be infected by religion and deity myth.
Other reasons for "anger" might include the righteous telling us we are damned, wrong, ignorant, etc., when in fact we are likely the furthest from ignorant anyone could be. When someone leaves a religion, a decision that large is not to be taken lightly, so research and deep thought is required.
And when these believers try to "save" you, they believe it's their duty, and the condescension and arrogance can easily trigger anger in the freethinker. When I recently told a family member of my atheism, I was asked if I had joined a cult, followed by the comment that it seemed like I had been indoctrinated. I, of course, made it clear that they were the ones who were indoctrinated and I had merely used critical thinking and reason to arrive at this point in my life.