After waiting some time, which I cut from the start of the film, the hover-fly took off from the umbel of cow parsley. This plant stood out of the wind, under an elder, next to a sunny wall. In the film you see the hover-fly take off and disappear from the frame of the film. But a second later it tumbles back into view.

I had not noticed that the fly fell back untill I saw the film, afterwards. I went back to the same spot and saw that the tip of a leaf of the elder was hanging over the umbel at a vertical distance of 25 cm, right in the flyway of the hover-fly. The fly probably had not noticed the leaf and this is what must have caused the fly to fall and tumble on its back on the umbel.