Prior to living in Sunnydale, Buffy lived in Los Angeles. After being expelled and the divorce of her parents, she moved to Sunnydale with Joyce, where Joyce now owns an art gallery.
Buffy arrives seemingly unaware that she has a watcher waiting for her (it could be theorized that she is just in denial).
From Welcome to the Hellmouth:
GILES: Can I help you?
He is British, of middle age, with a quiet intensity.
BUFFY: I was looking for some, well, books. I’m new.
GILES: Miss Summers.
BUFFY: Good call. I guess I’m the only new kid.
GILES: I’m Mr. Giles, the librarian.
BUFFY: Great. So you have, uh, --
GILES: I know what you’re after.
He leads her to the check-out desk by the door. His office can be seen behind it.
He pulls a book out and slides it toward Buffy. Huge leather bound, with a single word set in gild in the cover.
Real concern floods Buffy’s face, along with the understanding. She steps back from the desk, eyes on the librarian.
BUFFY: That’s not what I’m looking for.
GILES: Are you sure?
BUFFY: I’m way sure.
GILES: My mistake.
Buffy's Wikia page states that Buffy was predestined to move to Sunnydale, but is unsourced.
Little did they know that the town was located on a Hellmouth, and that Buffy's presence there was predestined.
The pilot episode also tells us that Giles was only recently transferred to Sunnydale
WILLOW: It has that effect on most kids. I love it, though. It’s a great collection, and the new librarian’s really cool.
The closest thing we hear to a reason for the move (aside from being expelled from school) is that Joyce "wanted to."
CORDELIA: Oh! I would kill to live in L.A. Being that close to that many shoes… Why’d you come here?
BUFFY: Because my Mom moved, is the reason. I mean we both moved. But my Mom wanted to.
So, with Giles having been only recently sent to Sunnydale in anticipation of Buffy's arrival, we can assume that the Watcher's Council either knew, or had reliable information that Buffy and Joyce would be moving to Sunnydale.
Even if their move was "predestined" as the wikia states, there is no particular guarantee of when they might move to Sunnydale. (Or is there?)
Was this move to Sunndydale influenced by some sort of machinations of the Council or any other force?