How to know if anybody change the db_owner in SQL server
Are you running any DMV capture tools like sp_whoisactive, redgate, DPA, etc or had tracing enabled? That will make it easier to see if it catches the SQL that ran making the change.

What I would do now is set up a trigger that will track the aspects of who makes permission changes to your database and log those transactions for future review. 

Example of DDL Trigger.