it could be that they didn't, or refused to, change something they wanted changed. friends of mine just sold their condo, their HOA was ordering everyone to fix their balconies different, nice 30k unexpected fee.
Most likely they were behind on paying HOA dues, if the fee was not part of their escrow. When people fall behind on bills, HOA fees are one of the things they stop paying. If fees were part of escrow, and they stopped making house payments, same thing. When people file for bankruptcy, even with a lien on the property, HOA's are usually very far down the list, and the balance due is expected to be written off. To collect, the HOA can sometimes file suit in court. Past due balance on HOA fees can also include interest, penalties and legal fees, which can add up. Homeowners who file for bankruptcy are usually 1 yr or more behind on HOA payments. I've been on my HOA board for many years.