Knock down ginger was the name of the game show segment in the short lived 1989 television comedy chat show One hour with Jonathan Ross. The segment was notable for introducing comedians Vic Reeves, Bob Mortimer, Charlie Higson and others to television. The comedians would be behind a number of doors and the contestants would knock on the doors and then face some questions from them.
Please see the answers offered to your identical question in Phrases & Sayings. I think it's pretty safe to say the game long predated anything Jonathan Ross did!
Is cos the first person to do it played it with a boy or girl who was ginger, and in their haste to get away from the door, they knocked down ginger? that is the best reason ever isnt it?!!