The reason - its the 65th Anniversary of Operation Chastise the Dambusters Attack. The attack itself was a amazing feat of flying and the use of the new Bouncing Bomb. A service was held with the last surviving member of the raid. The reason for the 'flyby' being there is that they used the Derwent Dams for practising (they didn't blow those up though).
The film, made in the 1950's, is one of the iconic films, partly due to the theme tune (Dambusters March). My current worry is that the film is being remade - Hollywood is doing an excellent job of making a mess of famous and iconic British films. I really wish they don't this time - but I'm not hopeful.