The wilderness farm spawns monsters at night by default and you can also have the option of having any of the farms spawn monsters if you turn it on when creating a new game.
I also know a few mods that spawns monsters, like the monster crop, the magical crops (the slime fruit can attract slimes) and so on, but I think it would be easier if you took a screen of your mods folder so we can check the likely culprit -- but I do have a strong hunch it might be the wilderness farm.
You can get rid of the monsters by unlocking the Witch's Hut and bringing a Strange Bun to one of the statues. I just started the forest map with monsters at night myself.