Johnson 85 hp blows up

Dad's boat motor blew up this week while fishing Toledo Bend in LA/TX. This motor had been rebuilt in recent years and was low houred. Just wanted to ask what could of happened. I know operating a boat and maintaining it properly consists of many do's and don'ts. Motor was a 1984 Johnson Javelin 85 hp. I do know we didn't start using ethanol treatment as soon as some people did but we have been using the treatment for a good while. Just wanted to hear of some reasons good motors like this just randomly blow sometimes. Thanks in advance.