I could not find any official sources on that, but I believe that the question "What started it" might be answered with: The invention of time travel.
Similar to how the time travelers cannot travel further than (what was it, 50? don't quite remember) years ahead/behind point zero (which seems to be a that's just how it is thing in the universe of the movie) I think that this time loop the protagonist is stuck in, is an anomaly (or is it a "nomaly" when inventing time travel? who knows...) that was created at the instant the humans of that universe "broke" physics for the first time.
We cannot find a "true" beginning, because there isn't one. From outside of the loop, the events unfold in a logical manner and all people that were not the protagonist are just passing through the loop unaffected. Even other time travelers might be just fine in the sense that they have travels with escape routes.
It is only this strange instant of the protagonist which is stuck forever in the loop, reliving it over and over again, while all other people pass through it.
At the end of the movie, the handler says:
Understand, you are more than an agent, you are a gift given to the world through a predestination paradox. You are the only one. Free from history, ancestry.
Which gave me the idea for this theory... But ultimately, we cannot know the answer.