Monsters never provoke OAs when it would be blatantly stupid to do so (such as when it might kill them, or when they realize that someone's OA could entirely negate their tactics).
On the other hand, when monsters are not bloodied, they will often provoke an OA if it would allow them to make an attack or action that is more "attractive" to them.
I play very heavily with the ideas that:
An unbloodied monster isn't really hurt yet.
The PCs are tougher than almost all opponents the monster has ever met.
That second one may require some explanation. Most monsters haven't met an opponent that is able to beat them. If they had, they would be dead. Thus, almost all monsters (even really smart ones) are going to assume that they are a match for the PCs. After all, they've always been right so far...
The monster that has met tougher opposition is probably enslaved by it or fighting to avoid it. Thus, these monsters have a choice of fighting the PCs (who might be able to beat them) or facing something which definitely can beat them.
I even explained this to some of my players, when they were surprised that they were able to gain entry to a duergar stronghold (I didn't have every single duergar hiding behind the parapets and shooting crossbows at the PCs until they died). I explained that the security the duergar had posted (orc guards and duergar guards) had always been sufficient to see off intruders, so why would the duergar think a handful of "heroes" could stop them?
This rationale makes provoking OAs the reasonable action of an overconfident (but reasonably overconfident) foe.