Sometimes we disagree most fervently with those who share most of our views because it's so annoying that we cling to those differences rather than go all the way and agree about everything. Those with different cognitive foundations, like secular humanists and conservative Christians are practically aliens in relation to each other, so those opposites are easily demonized and dismissed. But liberal Christians can seem all the more annoying to secular humanists because the differences seem so miniscule, yet they persist all the same.

I also think liberal Christianity is annoying because it's a sham that's functionally atheistic. Conservative Christianity is a sham, too, of course, but the conservatives or traditionalists lack the critical thinking skills to recognize the incoherence of their worldview. The liberal Christians often have no such excuse.