Why is the Mormon Religion sometimes viewed as a cult

One View
NO, the Mormon religion is not a Cult. Is the Catholic religion a cult? What about the Muslims, and the Buddists? It is just another religion that has its own beliefs that were influences by prophecies. Read the Book of Mormon. That is the only way to truly know that it is not a cult. =)
Another View
That depends on one's definition of cult. There are a number of Christian groups that define a cult as any religion that claims to follow Christ but denies the central doctrines of the traditional Christian faith as taught in the 66 books of the Bible. The LDS and a number of other religions that believe themselves Christian fit that definition.
See Apologetics Index: Cult Faq
Another View
Calling "The Church of Jesus Christ of Latter-day Saints" (LDS) a cult amounts to nothing more than name calling. The calling of the LDS church a cult was initiated by Walter Martin in the 1970s. The deceptive tactics of this man have been documented by the Browns in their book, "They Lie in Wait to Deceive." The entire book is on line and can be downloaded for free at the link below. The Browns' book includes all the original letters with notary seals and signatures to substantiate the claims. This same man wrote the definition of a cult that he applies to the LDS and other churches to classify them as cults, but the same criteria may be used to classify any church as a cult. If another church redefines the word cult and then uses their definition to classify the LDS church in this manner it is an attempt to discredit LDS as a legitimate Christian faith. Of course the LDS church doctrine is different from other churches - if not, why even have a different Christian church.
ANSWER
I'm not Mormon, but in my opinion the Mormon church- the Church of Jesus Christ of Latter Day Saints is NOT a cult. There are however factions and groups that have BROKEN AWAY from LDS that while THEY do consider themselves Mormons they do not belong to LDS for instance groups like FLDS that are cult like.

Technical answer

A cult, in the proper sense of the word, is a group that adheres to the teachings of a single individual (who is usually considered to have had some divine revelation). Most major religions began as cults (including Islam, Christianity, Hinduism, Judaism, and Buddhism, as well as the Latter Day Saints) and grew into full-fledged faiths. In this day and age, LDS is no more and no less a cult than any other major sect of the Christian faith.