On Tue, 15 Jan 2019, 15:16 Daniel Bensen <[log in to unmask] wrote:

> This is probably a stupid question,

I can't work out why you'd think that. I can't see where any
stupidity might be.

but what caused the /x/ sound in Old English to disappear in some cases,
> but turn into /f/ in others?

Dialect mixture; the spreading of (certain pronunciations of) words from
one dialect to another.  /x/ generally went to zero, but in some dialects
ux went to uf. Which dialects, I don't know; nor do I know if the story of
the spread of the f variants is known. Lack of time now prevents me from
seeking the details in books (or Wikipedia, which probably has an article
on it).