[abh19]

Had you ever set the Due Soon settings (in preferences) before?

I assume due dates are something that OmniFocus periodically checks for - like maybe every hour or so. Maybe your particular pattern of OmniFocus use was preventing that script from running. I don't know anything about it--just a guess.

My advice is see if you can reproduce the behavior.... And maybe put those critical assignments into iCal too, until you're confident your copy of OmniFocus is behaving properly.