The exact reason why some would call Morminism a cult depends on the person doing the labelling. I have seen people use the term for various reasons including ignorance and bigotry. However, some aspects of Mormonism contribute to the misunderstanding.
* Mormons reject many of the post-biblical doctrines of mainstream Christianity. Because Mormons see their religion as restored Christianity, they understand that it is different from those denominations that descended from the Nicene Creed and the Reformation. People label what is different.
* Mormonism takes a lot of commitment. Being a disciple of Jesus Christ was never meant to be a hobby, but the effort to put God first can make Mormons appear the same on the surface. You will find that there is a lot of variety underneath.
* Because there is no professional local clergy, regular members do a lot of the work that a paid clergy would do in other faiths. This limits how much some Mormons participate in their larger communities. Most Mormons make an effort to be active contributors in their communities, but are often very busy.
* Mormons are taught to be modest and chaste. They do not use recreational drugs such as alchohol, tobacco, coffee, or tea. They often avoid social gatherings where sexuality, drinking, or drug use is common. Though new Mormons are encourage to maintain their previous friendships, many relationships change. Mormons do not avoid non-Mormons, but they can give that impression.