The trajectory plugin seems to start the point at the selected altitude level in the tool (in this example, 150h) instead of at the altitude of the point (in this example, Surface). This caused the trajectory to start at the 150h wind vector, instead of surface and then it seems to drop back down to surface for the next sample point. Reducing the sample time (increasing number of samples) obviously helped, but everything up to the second sample is erroneous.