Why a book moves a number

Either to balance what it stands to lose, or because it has changed its own opinion. From outside those two look identical.

A move is a single observable event with at least two possible causes, and nothing visible distinguishes them. The book knows which it was; nobody else does.

Modern books are far more willing to hold a position than the folklore suggests, so 'they are balancing the book' explains much less than it used to.

That is also the expensive part. Every service claiming to tell you which cause produced a given move is selling an inference. The inference may be reasonable; it is not an observation.

What to do with it. Treat line movement as information about the market's confidence and not as information about its reason.