Over on t' Who blog, I just did a post about established monsters acting out of character for novelty value. And I could generic-ise that subject out a bit...
You've got recurring monsters. Orcs, vampires, Imperial Stormtroopers, tentacular abominations from beyond the known universe, mean kids, whatever. They recur, they are monstrous, they appear when you're a bit stuck for a monster, the players have fun with them, they go away again. So how to shake things up?
What do they do that they haven't done in the game? What would they not normally do? Don't wreck their raison d'être if you want to use it again, and you may not want to go for complete face turns, but having them act a bit weird due to some other influence may still add something. Stormtroopers are there to be zapped, so showing them assisting a disaster relief effort will not help next time you need to get zappy - but showing them running in terror from a weird monster-of-the-week would be just fine.
This trick works best once the recurring monsters are familiar enough that their odd behaviour is immediately obvious, and the mystery catches the PCs' attention. And that's what the monsters want, if they're doing it on purpose.
Obfuscating one's motivations mostly works for sentient beings, of course, things that have motivations that they can obfuscate.
Zombies are generally a bad example due to the whole "mindless" thing, although even they could act a bit oddly if you want to vary your zombie apocalypse - George Romero does it, after all. However, this might lessen the blank lack of individuality and loss-of-identity horror of the walking dead. But if you're doing a zombie apocalypse you probably have rival bands of survivors, rogue government groups and the like as well, so when they recur, have them act differently...
Your common or garden tentacular abominations from beyond the known universe have alien motivations, so having them do something unfathomable is par for the course. When the loathsome GHfm'safwe' (yes, I named it by hitting the keyboard twice) attacks the radio antenna at the edge of town it's no more bizarre than when it goes after the investigators directly, but they might be able to work out a bit about its actions from it. Maybe.
And equally, their unusual behaviour might be due to other factors. The Orcs sue for peace because their king has died and left no heirs. The vampires are attacking more because someone else is stirring them up or stealing their victims. The Stormtroopers are running away because of the big scary thing behind you. The leader of the mean kids falls for your sister.
And since this started with a Who post, there's the noble tradition that one of the smarter world-threatening monsters has seen the error of its ways for some reason.
Maybe it's even something the PCs did.