It is not an outright rejection of reason, per se; religion can string premises together in a valid manner, but the gripes most hold is that it lacks any content in its premises. For instance, to make the claim that god is omnipotent and lets the terrible malaise of atrocities and diseases pester the human race seems ineffably cruel. These premises seem so intuitively erroneous, to any perceiving human being, it cannot act as a prominent model to structure society in a humane manner or as a solid foundation for ethics.
The majority of religious believers are holding onto premises that are formally, by the consensus of academics and the most learned of subjects (however dubiously we want to treat the scientific disciplines), to be absolute bunkum. To assert an agent took it upon itself to allow all entities to exist and handpicked the transmission of blatantly defective alleles as an act of benevolence should bring up some qualms about the soundness of these premises. You want to come to the conclusion a deity exists and is benevolent (which you can do by the tool of logic) and then look around at the world you live in. Each permutation of most monotheistic faiths is an attempt at an absolute normative model that threatens people who live in a dynamic reality. It abandons well-supported, rigorously tested and reputable information from being incorporated into people's moral considerations because it proves contrary to the incantations of some addle nomads from Mesopotamia.
The antitheist movement has not been waged against spirituality by all its proponents; for example, Sam Harris practices many rituals associated to Eastern faith and argues that emotional awareness, or spirituality, is a part of the neurobiology of humans. Additionally, people like the vociferous Christopher Hitchens claims that we have this form of need of meaning and value and it can be define as spirituality; however, Richard Dawkins remains seemingly sceptical of such a proposition. Firstly, you are attacking the antitheist movements as being grounded on flaunting or championing reason. This is true in the sense that it believes an empirical dissection of reality leaves the question of an intelligent agent creating all entities to be, at best, deistic (if some form of deity exists, he is indifferent or uncaring; this is in staunch opposition to the omnipotent, omniscient and omnipresent concoction many extoll). Secondly, many antitheists are aware that violence and the like is inherently a part of the psychology of man; we are apes of sorts with a bigger cerebral cortex and have the ability to keep our history of bloodshed. However, the argument is that the absolutist nature of the model monotheistic faiths preach, is that it subverts reputable consideration of reality by science, denies the origins of man or the biodiversity of the planet and totes a very distinctly controversial substance dualism in regards to an afterlife – this afterlife subordinates the only defensibly existent life we live now; see Nietzsche’s amor fati argument.
The status of science is an interesting topic. We can tell by Kuhn’s model that it takes a crisis and the weakening of a theory by ad hoc or auxiliary theories to be a sign for the scientific community to re-evaluate the presuppositions of their disciplines. This is all true. Scientific theories are tentative theses; they are there to coincide with the revelation of more details by findings. I do not think science has a telos (an ultimate goal) that is “truth”; I think it seeks to be a problem-solver and to help humanity develop. This is a very evolutionary view of science, which accommodates the possible falsity of all models of science (something religion does not do with their models). Many therefore reject religion because it stunts this problem solving; think of the example of stem cell research and also think about how difficult fundamentalist lobbies protest abortion and try and argue living entities devoid of preference fit into a preference utilitarian model (they assume fertilized eggs want to be alive and assume foetuses want to be born, despite the fact they lack capacity for such at a certain stage of development with re: to foetuses). However, we have to listen to spiels about the sanctity of life as people die from potentially curable diseases, people who fit into the utilitarian preference model our legal system seems to be based on with regards to rights.
I would also like to state that I am not wishing anything against religious folk or even condemning their morality in entirety. I just wish to clarify some of the rationale behind the rejection of religion and why it seems so lacking in coinciding with the evolution of information that science seems to be fulfilling. It also allows for some atrocities to be committed that may not have been committed if this information was considered and the absolutist zeitgeist of nomads from the past abandoned for something a little bit more avant garde than adamic sin, serpentine meddling and wonderfully repressive views of sexuality.