https://www.cnet.com/news/spacex-falcon-heavy-core-rocket-lost-after-toppling-into-the-ocean/

The Falcon Heavy's center core is a modified version of the nine-engine Falcon 9 booster rocket. When a Falcon 9 lands on the droneship, it's attached and stabilized by a device known as the 'octagrabber' that grabs on to the rocket to keep it upright. Falcon Heavy's core doesn't have the same setup so it could not be nailed down. A satirical Twitter account set up for the Octagrabber jokingly remarked it had "nothing to do" with the loss. 

The SpaceX droneship is currently on its way back from the Atlantic.