When would you check the box "Decedent's estate" under A on form 1041?

Taxpayer is trustee for a trust that was originally revocable but became irrevocable at death of mother. Prior CPA prepared 1041 with Decedent's estate checked at top of form.  I thought that once the grantor trust becomes irrevocable the trust would be Simple or Complex.  Thanks for any help on this matter.