Suzanne S. Barnhill wrote:

See http://home.earthlink.net/~wordfaqs/BottomLine.htm

Thanks Suzanne, that's a nice article. It didn't really cover the points I
was struggling with, though. As I said, I had made sure that widow/orphan
and similar issues weren't at stake in my case. Neither were
keep-paragraph-with-next and so forth.

In my case, it was apparently really just an issue of Word calculating how
much space that one single extra line was going to require on the bottom of
that page. It turned out that some of the more esoteric settings under
OptionsCompatibility were playing a role. I had "Suppress top spacing",
"Suppress spacing before and after page breaks", and "Top align lines with
exact spacing" (or whatever those are called in English) turned on. That
apparently made Word measure required line hights in ways that weren't
obvious from the way they were displayed on the screen.

One other point I haven't quite worked out yet is the sinister role of the
"footnote continuation notice". My impression was that Word reserves
vertical space somewhere on the bottom of the page for such a continuation
notice, even if none is needed. Can't quite see why, but that's what it
seemed to be doing. I could work around at least some of the space wasting,
by setting the formatting of the continuation notice to the minimum allowed
height, a nominal 0.7 pt.

Lukas