The following is from the Vermont constitution:

"§ 39. [FORMS OF PROSECUTIONS AND INDICTMENTS; FINES]

All prosecutions shall commence, By the authority of the State of 
Vermont . All Indictments shall conclude with these words, against the 
peace and dignity of the State . And all fines shall be proportioned to 
the offences."

question:

For the 8% penalty (a fine), what is the "offence"?

(I have searched the Vermont statutes and have found no law requiring 
property tax to be paid by the due date.)

disclosure: I am not a municipal official