Wandering traders are spawning on blocks that mobs normally do not spawn on, like trapdoors, or in areas that make no sense, like deep underground. This results in traders and their llamas getting stuck in various areas, possibly blocking movement of other mobs, triggering redstone devices, etc.

In sample picture, trader has spawned on trapdoors or possibly on the roof of the building next door (smooth sandstone or cut sandstone blocks.) In other cases, traders and their llamas are showing up in branch mines at Y=11 or Y=12.