I'll offer a no doubt idiosyncratic, personal and possibly controversial opinion on the topic given in your title - "Do we know what prompted Orwell to write 1984?". The question contained in the body of your question is slightly different:
Did Orwell ever talk about what, specifically, motivated him to go from pondering on fascism to writing the actual book?
Of course Orwell talked about his motivations for writing the book, but
I'll try to be a bit more general than quoting him directly on those reasons.
I've been reading Orwell off and on my whole life. I grew up in Bombay, India. When I was around 12 or 13, a young visiting English teacher to our school read us the first chapter of "Animal Farm". I had never heard of it before. A bit later, I might have been 15 then, I borrowed and read 1984 from the school library. I remember that it really spoke to me. A bit after that I found copies of the Collected Journalism, Essays, and Letters from a local library and read and re-read them for a while.
Orwell was a prolific essayist and journalist. He also wrote a lot of letters. One can read statements about why he wrote 1984, but it's more interesting to attempt to trace his thoughts.
To put it in a nutshell, Orwell seems to have become convinced, over a period of about 15 years or so, starting in the early 30s, that totalitarianism and totalitarian leaders, which included the examples of his contemporaries Franco, Hitler and Stalin, posed a special and possibly existential threat to the human species. He also seems to not have been so much concerned with totalitarianism's directly destructive potential, despite horrible examples like the Nazis, so much as what they might do to human society. In other words, his concerns appear to have been at least partly humanistic.
Not only that, he seems to have felt there was far too much cultural acquiescence to totalitarian thinking among people at large, and not enough awareness of the dangers. His writings directly on the subject started in the mid 30s and increased in volume as time went on, with such themes eventually playing a dominant role.
Orwell's major writing is available online, some of it is out of copyright in Australia, and so is available on Project Gutenberg Australia, so it's easy to look through them and see these concerns taking shape.
His interest in this really seems to have started during the Spanish Civil War. As most people reading this doubtless know, Orwell fought briefly in the Spanish Civil War in an anti-fascist outfit called POUM and was wounded. He wrote a famous book about it called "Homage to Catalonia".
After that, his interest seems to have continued and increased as he thought and wrote about similar totalitarian phenomena like the Nazis and the Bolshevik/Communist Party.
A fairly detailed and explicit discussion of all this is in his essay: "Why I write" (1946), which includes the well-known passage:
The Spanish war and other events in 1936-37 turned the scale and thereafter I knew where I stood. Every line of serious work that I have written since 1936 has been written, directly or indirectly, against totalitarianism and for democratic socialism, as I understand it.
There is also a revealing bit of dialog in the 1939 novel "Coming Up for Air", which is full of references to Hitler and Stalin.
'I think you've got it wrong. Old Hitler's something different.
So's Joe Stalin. They aren't like these chaps in the old days who
crucified people and chopped their heads off and so forth, just for
the fun of it. They're after something quite new--something that's
never been heard of before.'
'My dear fellow! There is nothing new under the sun.'
The first quote is from Bowling, the protagonist. The response is from his friend, a school teacher named Porteous.
It seems that Orwell himself thought that Hitler and Stalin were "after something quite new".
Reading between the lines, Orwell decided in the 1940s to use a novel as a more effective vehicle to transmit his ideas and thoughts about totalitarianism to a wider audience. That novel was 1984.
If you will forgive some editorializing, it's interesting to note what Orwell did not consider to be a particular danger or problem. Specifically, standard Western Imperialism. Though he grew up during the British Empire, and even served as a British Imperial Policeman in Burma (and wrote a novel set there), he seems to have mostly tended to make excuses for it. For example, a random example from George Orwell Explains in a Revealing 1944 Letter Why He’d Write 1984 (borrowed from Bookeater's answer above):
I know enough of British imperialism not to like it, but I would support it against Nazism or Japanese imperialism, as the lesser evil.
So he didn't "like" British imperialism but would "support" it. This is despite the truly impressive crimes and destruction committed by the British and their cousin the United States. And people like Hitler having acknowledged their indebtedness to Britain and US policies as having "shown the way". But this is in keeping with general British attitudes on the matter, which, even in the 21st century, remain largely unchanged. And it's certainly true that Britain and the United States have never been totalitarian internally.
