It may easier to consider the airmass point of view. In a down-burst the airmass is descending, the aircraft has inertial flight path stability and basic aerodynamic stability; these together with any control input to maintain the desired flight path will be seen as a loss of airspeed and increasing AoA, but not instantaneously.
The incident reported below shows the magnitude of the airmass movement in a severe encounter and may provide a better understanding of an actual event.
Windshear Incident