New User field being assigned a value unintentionally

New User field being assigned a value unintentionally

In an application we are using the new User field. It is being assigned a value when a user changes to a record by a function. The field is not referenced in the function. Is this normal behavior. 

My understanding is this field works like a normal dropdown and is not changed unless selected by a user on the screen or is explicitly set with a script command. .