My guess is they agreed at some point to pay it i.e. cosign. Otherwise, debts die with you.
I thought creditors could go after debtors estate - assuming there is an estate.
Nope. Not true. Not even close.
Your estate is liable for your obligations after you die, in this order:
Funeral expenses. Federal Taxes. State Taxes. Municipal Taxes. Probate fees. Secured debt. Unsecured debt. Your heirs.
Your heirs come last, unless they've loaned you money under a secured debt agreement.
In some states with filial obligation laws, your children [or parents] can inherit filial obligations in the event the estate can't pay. That is the law and the case law in Pennsylvania. Don't like it? You shouldn't. Your sole recourse is to tell your legislature to repeal the filial responsibility law if your state has one.