Automatic narrowing in Emacs?

[David Sumbler]

This may be slightly off-topic, but it concerns a problem I have
encountered in Emacs when editing Lilypond files and at no other time.

I always use Emacs for creating and editing my Lilypond files.  Some of
these get quite large.  When editing a very long file (typically over
2,000 lines) I have discovered occasionally that I have apparently
inadvertently deleted a chunk at the beginning of the file.

I got in a terrible mess a couple of times trying to restore this
missing chunk from earlier versions of the file, only to find
afterwards that the relevant portion was never missing at all: it was
simply not shown in the window.

I now realise that in these cases the buffer has been "narrowed".  This
is a useful feature of Emacs that I wasn't even aware of previously,
and no doubt I shall start to use it intentionally in future.

But I find that a large buffer gets narrowed automatically when I am
editing, even though I have not given a command for this.  Even if I
then issue a 'widen' command (Ctrl-x n w) the buffer gets narrowed
again after short while.

I can find no reference in the Emacs Manual to this automatic
behaviour, so I thought perhaps it was a feature of the Lilypond Emacs 
mode.  But having looked at the files in my
/usr/local/lilypond/usr/share/emacs/site-lisp/
directory, I cannot find anything relevant there either.

Can anyone explain why this odd behaviour is occurring?

David