Why is laravel changing the name of the column?

Here is the error I received:

"SQLSTATE[42S22]: [Microsoft][ODBC Driver 13 for SQL Server][SQL Server]Invalid column name 'user_user_id'. (SQL: insert into [APPLICATIONS] ([app_nm], [app_path], [app_readme_path], [app_manual_path], [is_url], [user_user_id]) values (TEST, uploads/2240120001.jpg, uploads/50225114.5-Data.pdf, uploads/9275171481557696855-FixedDeposits.docx, 0, 1))"

I have 3 Models:

User.php

public $primaryKey = 'user_id';

public function posts()
{
    return $this->hasMany('App\Post');
}

public function roles()
{
    return $this->belongsToMany('App\Role','USER_ROLE','user_id','role_id');
}

Role.php

public $timestamps = false;
public $primaryKey = 'role_id';

public function users()
{
    return $this->belongsTo('App\User');
}

Post.php

public $timestamps = false;
protected $table = 'APPLICATIONS';
protected $primaryKey='app_id';

public function user()
{
    return $this->belongsTo('App\User');
}

Laravel adds 'user_' to the 'user_id' column name resulting in the above error. I have checked the laravel 5.5 api to see if there is an additional parameter that needs to be supplied but that doesn't seem to be the case.

What would cause this concatenation and how do I fix it?

via Chebli Mohamed