It's not the first time I have heard of that happening. Guys will get rigged up with a stand-up harness and put a lanyard to their rod because they are concerned about losing their rod, but fail to put a lanyard from the harness to the boat. Once you are in the water with even a mediocre fish, the tension on the line keeps you from getting the lanyard unsnapped.
On game reels it is very easy to pull the lever to free spool even if the fish drags you underwater . That hopefully will give you time to undo the harness hooks before you get spooled .
Unless you have lost grip on the rod as you try to save yourself from going overboard and the rod is pulled to the extention of the lanyard at which point you probably could not reach the reel. It's just an awful thing to think about that happening to yourself.