prevent automatic minimizing

When keeping multiple documents open at once--and without minimizing any of
them--I find that after a little while Word automatically minimizes them so
that when I press Ctrl F6 a minimized doc appears, requiring an extra action
that I resent. How can I stop that intrusion and see a full document each
time I press that command? I don't remember that happening before Word 2003.

Thanks for all you do!