I've experienced this throughout the game, and always thought, I need to tell someone that. Well anyway, now I am. During broadside battles I'm trying to engage ships by myself but then I get to about half hp and another ship joins in and I have to retreat to the windlane. I'm experiencing in especially now in Port Regal.
Yup they do love to swarm on ya. That will happen a lot. It is where you have to learn the sneaky part of pirating and find someone out by themselves and stalk in and get them. Where you position yourself matters too. It is possible to keep your target between you and the other ships and if you keep at a yellow distance they tend to not notice you as fast. Just slip your ship into Tiptoe gear and become that stalking hunter that you have seen on Animal Planet so many times .