A reasonable size loose article (for example a Mobile Phone) could be enough to dislodge the Sear in the Firing Unit and initiate the ejection sequence.
For all its high-tech cleverness, the M-B seat has never been as totally 'safe' with pins fitted than was the Folland seat in the Gnat. That used a physical interruptor plate which prevented anything striking the cartridge and firing the seat when the lever was at 'safe'....... Simple and totally reliable.