There cannot be one straight cause of religious conflict. However if we want to understand what triggers religious conflict, it is ignorance. smart people ride in the ignorance of the majority to whip religious sentiments in order to achieve their set agenda.
I think, the cause is ignorance of the teachings of other religions and the attitude of assuming that others are wrong, and lacking respect for the beliefs of others
Often religious conflicts are stoked by states for a political aim. For example, during the Cold War, when the Russian military had a physical presence in Afghanistan, religion was used as a reason for the mujahideen to drive the Russians out of Afghanistan.
I would say it is because of Egocentrism---- thinking only about yourself and what is good for you (Cambridge English dictionary). Thus, the basic thing for the religious conflict is inability to differentiate between your belief and other others, apart from that, ignorance of human nature. We may differ in skin color but we share same red color blood (no green blood is appearing yet). Similarly we might be religious or scientist, we both believe in reason. Here it is the supernatural that we call it in different names. I want to windup my comment with the ever shining quote of Napoleon Bonaparte “The world suffers a lot. Not because of the violence of bad people, but because of the silence of good people."
Though there are some innate behavior as we all know, majority of our knowledge are learned behaviors. When we say learning it is a relatively permanent change in behavior that brought about by experience. Well if this is what learning is, definitely egocentrism is a learned behavior and so do religiosity.
The question was what can be the cause of religious behavior?
I believe that no one can dare to say it is b/c of the nature (it occurs naturally). If I must to say we human being come to this earth with little knowledge otherwise with no ink like a white plate. People teach us how to act in childhood time then grown up in the societies mood.
If you cultivate a tree in the proper way you will get a straight and probably perfect one. If a tree is treated improperly then it bent out the way we give it.
Likewise, egocentric is a learned behavior. We used to say “ours is best” “they are this this and blabla things” and so does same while talking in religiosity
People’s delusions, ideas, and creeds themselves have no influence on reality which is in line with the CANI's IMPERATIVE of WORLDVIEW. So they cannot be injurious to the public, provided that they don't lead to indoctrination, dominance, or subordination of others to meet own egoistic needs.
Unfortunately, religious ideas have become a diabolic tool to control and use people by both authorities and clergy. Religions, particularly organized ones, including Christianity and Islam, have been shown to be hazardous, harmful, and often deadly for humans. Directly or indirectly religions have been the cause of conflicts often leading to wars in human history and they're responsible for the death of hundreds of millions of people around the world. Religion drives apart people of opposite views, inspires mutual loathing and hatred, and is regarded as a threat to human rights.
Do the centuries-old conflicts and hatred, inspired religiously, indeed result from the substantive differences between professed ideas or other reasons?