An Azure relational database service.
No, that did not happen automatically. Someone ran some commands. The user may not have explicitly execute ALTER TABLE commands, but it may have happened implicitly because of some operation.
If you did not have proper auditing in place to capture DDL commands, it's difficult to find out what happened.
The easiest way to track such events is to set up an DDL trigger:
CREATE TRIGGER ddltri ON DATABASE
FOR DDL_DATABASE_LEVEL_EVENTS AS
INSERT ddlaudit(evendata, time, username)
SELECT eventdata(), sysdatetime(), USER