How do writers explain their initial flashbacks when they're writing a novel, movie, or something l?

That's an interesting question. Writers often find that the information provided in a flashback, which essentially makes it part of the exposition, is better placed later in the story because it is, frankly, less exciting. (Before I go on, does this make sense to you?) Additionally the background information can be much more interesting when it helps to explain a situation that has already captured the readers interest.

I hope this helps! :)