What practice caused Mormons to be persecuted in the late 1850s?

There were many reasons why Mormons were persecuted in the 1800's. Essentially, it is because they were seen as a threat to the typical American Christian way of life at the time as well as because of their political ideas. Here are some reasons Mormons were persecuted in the 1800's:

-They were anti-slavery. This was especially bad for them in pro-slavery Missouri, where they were ordered out on extermination order from the Governor who made it legal to kill a Mormon.

-They believed in a "new" religion. While Mormons believe their religion is a restoration of ancient Christianity, traditional Christians of the time did not like that Mormons differed from Christian norms. They had a modern prophet, additional books of scripture, and other beliefs that simply didn't exist in mainstream Christianity.

-They believed in the concept of 'gathering'. Mormons were encouraged to gather and live near each other, which resulted in them nearly overtaking small towns. (Imagine the population of your town doubling when a bunch of outsiders from a weird religion move in all at once!) Residents fought back in a struggle to maintain their identity. Some made it illegal for Mormons to vote or hold public office to preserve their preferred political climate.

-In some cases, the Mormons believed the land they were in was a "promised land" given to them by God, and could treat those who already lived there unkindly.

-Often, Mormons would favor going to stores that were owned by other Mormons rather than using goods and services offered by non-Mormons.

-Mormons had some practices viewed as ungodly or immoral by outsiders. This included the practice of polygamy (having more than one wife) which Mormons believed was in accordance with a Biblical restoration. Also some Mormon communities lived a communal lifestyle "having all things in common" as the New Testament describes. Mormons were also known for enjoying music and dancing, even in worship, which was forbidden by many Christians at the time.

-Mormons were also seen as being very liberal. Once in Utah, Mormon women were allowed to vote, they were encouraged to have jobs, were given a voice and responsibility in the congregation, were sent to universities, established hospitals and schools, and even wore short skirts with pants underneath! They assisted and worked with the native tribes, had a good rapport with most racial minorities, and encouraged their people to gain both secular and religious education.

-Finally, some people who persecuted Mormons were simply misguided or uninformed. Rumors were rampant in newspapers of the day, saying that Mormons were secretly devils and had horns to prove it, Mormons were money diggers, that they had satanic practices, they operated a secret mafia that killed defectors, that they kidnapped young girls and threw them off the pinnacle of the temple if they refused to marry. All these things were, of course, untrue, but the people had no way to know that.