It could be that Word runs out of memory. It's difficult
to anticipate when it will happen because it depends on
how big the doc is, how many tables are in it, how many
levels of undo it's working with, how much repaginating
it has to do, the other applications you've got open etc
etc. Sometimes it will tell you: Word is out of memory,
and then you can just reboot, but more often than not it
just dies.