Why dbt creating schema name which I have not used anywhere in code?

I have seen that the creating the schema by it own means other than schema which I have mentioned in code.
It has happened twice. First it has created schema with name “default” and next time it has created schema name “SAMPLE_100K_AU_MI”. Sometimes it adds the tables as well in this schema or sometimes not.

My question is why this happening and how to avoid it to happen?