Cured villagers randomly leave their beds at night

I built some houses for some villagers I had cured, and tried to specify which one would reside in which. This took soom doing, but eventually worked out. The following night, two of the villagers I had built specific homes for promptly got up and went for each other's beds, completely ruining my plans. It doesn't seem to be at a specific time either, as it can happen multiple times per night.