Let us begin with a model of a civil suit in tort. That is false. Suppose the plaintiff has sunk most of his costs up front. The plaintiff might want to maintain a reputation for carrying out threats. The essential requirement is that the plaintiff expects to be in future litigation and is willing to take a loss on the present case to preserve his reputation.
Mistakes always occur.
I will only mention two of the many reasons for mistakes. The plaintiff faces an agency problem, because he relies on an attorney's advice. One-third of zero, plus a reputation for losing cases is not attractive to ambulance chasers. The plaintiff may value the publicity, as is commonly suspected of public prosecutors who bring sensational but legally dubious civil or criminal suits. The plaintiff may value the delay that litigation brings.