From $120 to $2000, that is really bad. Some would say usurious.
I'd assume the contract had a clause saying they'd be responsible for the legal fees involved in collecting the debt if they refused to pay it voluntarily. The HOA most likely had money out of pocket to pay for the necessary paperwork to collect the money the couple had previously agreed they would pay. Seems fair for the couple to have to pay for the extra cost resulting from their refusing to pay the bill when due.