Check the bottom board for dead varroa mites, it will be quite evident if the varroa population was significant enough to be the cause.
It may be the reason if the owner have not done nothing with mites.
If there is apistan resistant miten population, so it happens to hive even if you give strips.
Who knows. There is many reasons, and nothing has gone wrong. Some things are just normal.
That is not normal that everything goes all right!