I don't understand it either but most of it is actually a design choice. EA choose to code it so that in some situations, townies are assigned random outfits. Those use a certain outfit style, which would be perfectly fine if EA were carefully enough to assign only fitting outfits to that stye. Unfortunately, when new assets are created for the game by EA, they sometimes tend to forget about the outfit styles and don't adjust them properly.
Some of those outfit problems are however bugs, like for instance some of the outfits at the Geek Con which are the result of broken costumes. If a costume is broken, the sim will wear a completely randomized outfit instead.
Another example are the festival shirts. If a sim chooses to wear a festival outfit and is currently wearing a full body outfit, this action will render him/her bottomless atm.
There are fixes for all broken outfits at my site, but there are really a lot by now and I don't get why EA are not giving up that poorly design choice, but even add more of them with almost every pack.
@LiELF: I just noticed that you mentioned the ridiculous outfits occured at night. There's another possible reason for this, the fake vampire outfits. There's a fix for them in the Vampire section of my mods.
