The cloud moves horizontally at X speed, dragging this whole contraption with it.
Imagine seeing this coming for you
That may not be what happened to RM though. An individual cell in clear air is easy to see and avoid - both on radar or with eyeballs. Flying or sailing, the more dangerous ones are embedded in a general area of storms and much harder to spot. It also does not have to be one little microburst. I think the "Derechio" is the name now given to a whole giant line of this stuff instead of one localized phenomanon.
But what's carrying the leafblower? The wind. At a speed much lower than the wind.
Maybe Rimas is in charge of wind?