At the beginning of his speech, why does King explain his reasons for speaking out against the Vietnam War?

The question does not say which of Martin Luther King's speeches you are referring to. However, by far his most famous speech about Vietnam was his first, given on April 4, 1967 (one year before his assassination). In this speech, given in New York City, he broke what he called "the betrayal of [his] own silences" over the war, and he begins the speech by listing the reasons he is opposed to the conflict. These include his conviction that the war diverted resources from the Great Society, the fact that a disproportionate number of black and poor Americans fought in the war, and simply that the war was inconsistent with his Christian pacifism. He is compelled, perhaps, to give the reasons for the positions he has taken because he does not wish to have his message misconstrued as radicalism. He is careful to make connections between his Christian faith, his struggle for racial equality, and his opposition to the war. He explains himself in the hopes that his message for peace in Vietnam will not weaken his message for racial justice in America. He outlines the concerns of his critics in one revealing passage:

At the heart of their concerns, this query has often loomed large and loud: “Why are you speaking about the war, Dr. King? Why are you joining the voices of dissent?” “Peace and civil rights don’t mix,” they say. “Aren’t you hurting the cause of your people?” they ask.

As mentioned above, King outlines the reasons (he lists seven) for his position in an attempt to make the connection he perceives between civil rights and Vietnam.