It happened on his 72nd birthday - March 14th, 1951 - and is supposedly the result of his having already posed for umpteen photographs that day. He was tired of it and, having been asked to smile yet again, stuck his tongue out as a sign he was - almost childishly - bored with the whole procedure. Click http://www.mentalfloss.com/archives/archive2003-03 -14.htm
for more information.