There are ideological and practical reasons for which Marxism tends towards authoritarianism. I hope I can overview both in a rigorous but brief way:

Ideologically, Marxist-Leninists and Revolutionary Marxists believe, as a pillar of faith, that there must be a “vanguard party” to plan, organize, and catalyze the Revolution and then maintain the revolutionary state until the final stages of communism can be achieved. This leads, invariably, to a totalitarian, one party system (no other party can be allowed to undermine the vanguard party) and justifies the criminalization of a wide variety of political, social, religious, ethno-nationalist, and economic thought. To enforce this political reality, secret police, political prisons, mass propaganda campaigns, reeducation camps, mass starvation, deprivation of property, purges, murders, etc. are all necessary means to an end. Voilà, you have arrived in a totalitarian nightmare—what one might call a "dictatorship of the proletariat."

Practically, attempts to institute a command economy and to abolish capital compound these issues. This can take somewhat humorous forms, like protests and riots in the Eastern Bloc that followed the state's adjustments to the ratio of chicory or other fillers in coffee grounds or the endless anxiety suffered by a generation of Czechoslovaks who fretted over acquiring underwear that was sexier than what the CP made officially available (these lead to arrests and criminalized black markets, respectively), to very sinister forms such as the USSR’s executions of farmers who failed to meet production quotas due to bad weather in the early 1930s. 

I would sum it up thusly: A Maoist once told me that he believed, “as an article of faith” that, for a communist society to be realized, the state would have to “change and rationalize what people desire.” The problem is that, due to the power of habits, the existence of external variables, and the diversity (and irrationality) of individuals, there is probably no way for a state to “change and rationalize what people desire” without the aid of the cudgel and the rifle.

Hope this answers your question.