Why does Microsoft Office 2011 open automatically upon start up? Why do Microsoft Office Word, Excel, et cetera open automatically upon login although I've disabled the 'Open at login' feature?
It sounds like you had these apps open at last shutdown or logout. If you don't clear the check mark in the popup that states "Reopen windows when logging back in" all of your previously running apps will restart. I say this because in Lion sometimes that checkmark would not "stick" and default to on, and you would have to check it off at every shutdown/login, and you might have missed it.

Investigate this as well: Go to the "Users and Groups" System Preference pane. Click your account on the left (should already be highlighted and on top as current user). Make sure that under the tab "login items" that no Office apps reside there and are checked.

There are some other things to try but I believe these answers will cast light on your issue.