As seen from behind, all the props spin anticlockwise. The wingtip vortices spin anticlockwise from the right wing, clockwise from the left. The interactions between all that spinning air are obviously complex, but the outside right propeller, at least, should enhance the right wingtip vortex, as they are spinning the same way. The wind probably also plays a part, and was light and from left to right. The right wing vortex develops to look like an aerial tornado, only with a narrow “tube” of smoke trapped in the low-pressure centre, or core, instead of dust and debris.