A stall occurs when the smooth airflow over the unmanned airplane's wing is disrupted and lift decreases. This is caused by

Why →A stall is an aerodynamic event that occurs when the wing's angle of attack exceeds the critical angle (approximately 15-18° for most airfoils). At the critical AOA, airflow separates from the upper wing surface, destroying lift. Stall can occur at any airspeed, even at high speed in an aggressive pullout.

The trap →Low airspeed does not itself cause the stall: exceeding the critical angle of attack does, which is why an aggressive high-speed pullout can stall a wing. Propeller RPM has nothing to do with wing stall.