What influences in Orwell's life led him to see totalitarianism as such a threat to humanity?
You want biographical influences? Honestly, I don't think there are such influences in his early life. However, a concern with the nature of the state and power permeated a great deal of the intellectual life of the twentieth century, and many people had similar concerns (if not such firm conclusions). Look at how many writers and artists have been involved in politics at some point: Hammett, Rand, Hemingway, Huxley, Leguin, Conrad, Warren, etc.
I'd say that his views are shaped in part by the time, and in part by his personal studies. However, if I had to point to direct experiences, I'd say his time serving in the British foreign service in India showed him the dangers of power and especially formally structured power.
One of the largest was the rise to power and the way that Stalin used his power to create a powerful totalitarian state on the back of the deaths of millions and millions of people in the Soviet Union. Having also seen the aftermath of Hitler's rise to power and the holocaust, one can understand a reasonable fear of totalitarianism and the results of its emergence.
This, combined with his concern for the poor and the working class and the feeling that socialism, in which the resources and their distribution are controlled by the government, combine to make him think that totalitarianism is the worst of all possible options.