An unproven claim has been made that British Intelligence assassinated Prince George.
The appeasement movement, involving many of the nobility and the powerful- even the [then] newly-abdicated King, Edward VIII (George's older brother)- were purportedly Nazi sympathisers.
One possible reason is given by author Charles Higham, in the second, revised edition of his book The Duchess of Windsor:The Secret Life, as serious concern over the Duke of Kent's lack of discretion and his political dealings with Nazi leadership, with negotiations towards a separate peace, to allow Germany to concentrate on its war with the Soviet Union in eastern Europe.
Higham writes that the Special Operations Executive (SOE), worried that the Duke would talk about these matters once he left the British Isles, tampered with the plane before its takeoff, ensuring its crash soon afterwards, with the deaths of all but one of the passengers.