In Outlook, with and without the Word editor:

The ghost in my system keeps selecting text for me: I can
be just sitting here, minding my own business, typing any
old thing, and happen to leave the shift-key depressed
longer than is absolutely required.

An unwanted selection begins at the blinking insertion
point, and ends wherever the pointer "I-beam" happens to
be at the moment.  It stops exactly at the I-beam, and
then includes the entire word (I have Word set to select
full words, so maybe this does follow some rules).  

Yes, you are right: it is as if I have been selecting by
the "click (at the beginning) - shift - click (at the
end)" method.  But I did NOT click at either end!

I could work around this, except that.

When I release the shift key, and even press the "ESC"
key, the selection does not become unselected.  Even
clicking the mouse also does not de-select it.   I just
have to wait for it to get ready to behave.

When I get tired of waiting, I click Ctrl-Alt-Delete/Task
Manager, all programs indicate "running." I hit ESC,
returning me to the document, which now shows no
selection, and all is well for a time.

Sometimes I have to invoke adult supervision (the Task
Manager) multiple times before it releases.

Other open programs are also frozen during this time.

Except for freezing up everything (!!?), this could be
useful, so perhaps it is a corrupted sub-set of commands
used for something.  I considered accessibility, so tried
installing and uninstalling MouseKeys; it's the same
either way.