I have no idea why the name Michael was chosen, but it appears he was originally known as "Prince George," at least for a week following his christening.
The caption accompanying a photograph of his christening, published in the The Times on August 5, 1942, refers to him as "Prince George." In its August 13, 1942 issue The Times announced that [FONT="]“The Duke and Duchess of Kent have decided that their younger son, who at his baptism on August 4 received the names Michael George Charles Franklin, shall be known as Prince Michael.” Other British newspapers published on that date state he would be called "[/FONT][FONT="]“Prince Michael, and not, as originally planned, as Prince George.”[/FONT]