for me it was easily the last game I played. I had 9 trade routes up at the time, and all of a sudden, BAM, I had 4 cargo ships plundered. it was not within my view, so I had to send a ship out in that direction, next turn, BAM, 2 more cargo ships plundered. I finally make it out there, and there are FIVE barb ships going OFF on my cargo ships.
so random. I never let my capital get hit, as my first 2 units do a quick scout then I leave 1 to protect from that point on. but lately I have added to this, putting a caravel on each of my key trade routes.