What could have killed me?

An interesting thing just happened to me and I'm curious to find out exactly what. I was just doing a lost shipment voyage, when about halfway in I got a call and needed to take a ~20 minute break. I had only been playing for about half an hour, so I wasn't too worried about sinking. I figured there'd be a pretty good chance to be sunk by another player by the time I got back, but I just parked my ship where I was in the middle of the sea and put a weight on my "W" button so that my pirate would continue to walk against the railing of the ship and not time out.

When I get back, I see that I'm on the ferry - no big surprise there. However, after I step through I respawn on my ship, still parked where I left it. I wonder if I came back in the middle of an attack, but I don't hear anything. The ship is quiet, there's is no water on board whatsoever and no visible damage anywhere. I died, but my ship didn't get scratched.

I'm curious how that's possible. Couldn't have been a thunderstrike, otherwise my ship would've let on rainwater and possibly fire damage. Another player of course could've killed me, but that seems highly unlikely - I had some loot on board and nobody stole anything, I also didn't see any ships close by.

Did I maybe get sniped by sirens? I was standing near / walking against the wheel & railing of my sloop when I went afk.