I understand the protestant reformation is the cut and dry answer, but I'm interested if there where some more in depth causes. From what I've learned, any discussion on the reformation has to include Martin Luther. However, europe must have been primed to accept his ideas? Had social discontent been prevalent or was it spontaneous?
For example; you could say WWII started in 1939 via Hitler, but if you really wanted to understand the start, you'd have to go back quite a bit further.
You are correct, you have to go back a bit to understand the context of the Reformation which in turn sets the context for the wars of religion. It is hard to distill the debates and schools of thought into a good answer in the space of a reddit postm but I will do my best. First, one of the biggest points of discussion regarding the Reformation is the question of just why it did take as widely as it did. The traditional answer has been that the laity of Europe, fed up with the abuses of the Catholic Church, abandoned it for the religion of Luther. However, this is an inadequate answer because it cannot explain why half of Europe remained in the Catholic fold. (it should be noted that many most likely accepted or rejected the reformation based on personal piety but this is difficult for historians to examine so temporal issues become the focus)
In response to this inadequacy, historians began to look at the social factors which may have led people to follow the Reformation or not. One idea, which will play into the later religious wars, is that the Reformation flattered the laity by taking power from the hierarchy of Church and bringing it to the people. No longer was the sacrament of communion something for the priest alone or the scripture and homily obscured from the common man. This theory argues that the reformation appealed to the peasant because it gave them access to religion. The problem here is that this idea only applies to the response of the uneducated lowest class of society to the Reformation.
In the case of the upper classes the most cited reasons for following the Reformation, aside from piety, is that by endorsing the new religion they could gain some kind of political or economic benefit by removing the authority of the Catholic hierarchy.
This is a very over simplified rundown of some of the social contexts behind the Reformation, but I want to get to the main question of the Wars of Religion. Here historians are divided into two main schools of thought, the first that they were completely motivated by religion, and the second that they were mainly political conflicts under the guise or religious conflict. The side that you come down on will be heavily influenced by how you understand the reasons for the Reformation taking hold in the first place and the particular regional conflict you are looking at.
For example, in Germany the peasants' war is often considered to be a result of a politicizing of Luther's theology. The thinking goes, no need for priests because all can read and understand the Bible for themselves = no need for any social control from princes. In France, the wars of religion have a more religious tone but are still political. The crown and church were closely united and the Church desired the protestant heresy to be destroyed while the king wanted to assert his power over the nobility.
Again, this is a very quick rundown and the issues are much more complicated and nuanced than i have presented.
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