Shares of Rivian Automotive Inc. RIVN, -12.30% tumbled 16.3% toward a record low in premarket trading Monday, after media reports that major shareholder Ford Motor Co. F, -1.91% is planning to a large chunk of its stake in the electric vehicle maker. CNBC reported over the weekend that Ford was selling 8 million shares of Rivian, as the lock-up agreement, which required pre-initial-public-offering investors to hold onto their Rivian stock for at least 180 days, was set to expire on Sunday.