Funny enough I got this question because of game of thrones, but I've been thinking about it and the only answer I can come to is ships being boarded. Is this correct?
I don't want to discourage other replies, but a good thread back in '13 asks the question you're asking, specifically a reply by /u/thegodsarepleased found here!
I wrote an answer about this a while ago that may be of interest:
If you have follow-up questions, let me know -- I'm traveling today, but can check back in in a day or so. Thanks!