Documentation for selecting table name returning csv of the row
For each table that is created, Postgres creates a corresponding composite type: 


  Whenever you create a table, a composite type is also automatically created, with the same name as the table, to represent the table's row type.


When you use the table name in the select list, you are essentially using a single column of that composite type with several fields as part of the record (this is a good example on the difference between row and record and column and field).

The "CSV style" just happens to be the default text representation of a composite type.

A row constructor is closely related to this.

This is also the reason why select (a,b,c) from some_table is something different than select a,b,c from some_table in Postgres.