Helaman 4:12

and it was because of the pride of their hearts, because of their exceeding riches, yea, it was because of their oppression to the poor, withholding their food from the hungry, withholding their clothing from the naked, and smiting their humble brethren upon the cheek, making a mock of that which was sacred, denying the spirit of prophecy and of revelation, murdering, plundering, lying, steeling, committing adultery, raising up in great contentions, and deserting away into the land of Nephi, among the Lamanites;

It happened because of the pride in their hearts. It happened because they had too much money. They hurt the poor. They kept food from hungry people. They kept clothes from people who had none. They hit their humble brothers on the face. They made fun of holy things. They said there were no messages from God. They murdered, robbed, lied, and stole. They broke marriage promises. They caused great fights. They ran away to the land of Nephi to join the Lamanites.

It was because of their pride, their excessive wealth, and their cruelty to the poor—withholding food from the hungry, refusing clothes to those who needed them, mocking their humble brothers, making a joke of sacred things, rejecting prophetic revelation, murdering, stealing, lying, committing adultery, stirring up conflicts, and deserting to join the Lamanites.