What is the underlying issue that influenced the formation of the Essene religious group?
It is hard for us to trace the roots of the Essene group. As far as I know, its history is not well known at all. We know quite a bit about the history of the Pharisees, a bit less about the Saducees, but very little about the Essenes. Josephus mentions this group and his histories are generally fairly reliable, so we can probably assume on his authority that they existed. We also have the Dead Sea Scrolls, which were probably deposited in the caves near Qumran by an ascetic Jewish group living in Qumran. This is a reasonable inference, but is not proved. The most common theory is that the Essenes occupied the site at Qumran, but there is some debate about this. If the mainstream of scholars is right (and this is a big if) then the Dead Sea Scrolls represent the teachings and theology of the Essenes. They were in opposition to the Saducees and their control of the Temple. They had a strong apocalyptic tendency, talking about a Messiah-like figure quite a bit. Their first leader may be the one referred to as the "Teacher of Righteousness" in a number of the Dead Sea Scrolls. One of the most famous of the Dead Sea Scrolls is the War Scroll, which describes their apolyptic view. From the evidence at Qumran, we are pretty sure that they had a very strong emphasis on ritual purity and cleansings. There is evidence that they did a lot of ritual cleansing in water. This was a conservative group, with strong ascetic tendencies. Only men were admitted. What was the initial motivation for this group? Presumably it was not unlike many other historical ascetic movements. Probably the leaders saw the Jewish practice in Jerusalem to be very worldly and corrupted. Probably they wanted to move Israel back to a more spiritual faith. Theirs was a call to a more pure lifestyle. Theirs was a strong sense of a kingdom of God about to come. There is a lot of information about the Essenes out there. The article on Wikipedia is actually fairly good.