Executive officials are regularly called before Congress to testify as to why they made certain decisions. Why shouldn’t judges as well?
After the Supremes found FDR's National Recovery Act ("The Blue Eagle") unconstitutional, Roosevelt threatened to pack the court; i.e., to expand its number to 13.
The Supreme Court evidently got the message. After that, they relied on the Commerce Clause to expand Federal power to anything and everything.
Still do.