On the second flight of the Falcon Heavy, they successfully landed the side cores at LZ-1 and LZ-2, and the center core downrange on the ASDS Of Course I Still Love You.
Reports have come in that due to high seas, the stage fell off the barge.
It is a 100+ tall, 12 foot wide cylinder, standing on a large flat barge, so no doubt waves are very disruptive and toss it around.
Previous landings they have attached cables between the stage and welded attach points on the deck.
In the Thaicomm mission the booster landed hard and bent a leg and you can really see how they held it down in this image.
Later, they added the Xoomba/Octograbber.
This device is a Roomba shaped robot that drives out, in between the legs and grabs the stage, adding mass and lowering the center of the mass so the stage is more stable.
You can see how they used it here in this image.
It seems they did not use either approach on this landing. Do we know why?