The other two reasons were Britain's involvement in "the Nigeria/Biafra thing" and its support of the USA in respect of the Vietnam War.
Wikipedia points out that Lennon's gesture "had no effect on his MBE status, which could not be renounced. The medal, together with Lennon's letter [to the Queen], is held at the Central Chancery of the Orders of Knighthood."
© Haydn Thompson 2020