As a rule, you should explicitely specify
dbo schema if you want to create the object in this schema.
As you are
db_owner, your default schema is
dbo, so there is no problem when you don't specify dbo schema while creating objects. But for other (Windows) users it's not the same.
Your users are members of
Windows group that has no default schema. In this case the corresponding user and the schema are created when the user creates any object, it's documented here: CREATE SCHEMA (Transact-SQL)
Implicit Schema and User Creation
In some cases a user can use a database without having a database user
account (a database principal in the database). This can happen in the
A login has CONTROL SERVER privileges.
A Windows user does not have an individual database user account (a database principal in the database), but accesses a database as a
member of a Windows group which has a database user account (a
database principal for the Windows group).
When a user without a database user account creates an object without
specifying an existing schema, a database principal and default schema
will be automatically created in the database for that user. The
created database principal and schema will have the same name as the
name that user used when connecting to SQL Server (the SQL Server
authentication login name or the Windows user name).
This behavior is necessary to allow users that are based on Windows
groups to create and own objects. However it can result in the
unintentional creation of schemas and users. To avoid implicitly
creating users and schemas, whenever possible explicitly create
database principals and assign a default schema. Or explicitly state
an existing schema when creating objects in a database, using two or
three-part object names.
To resolve the issue just assign
dbo schema as
default schema to all yor users-Windows groups or write the schema explicitely when creating objects. Always.