Pressure at the intake of the compressor is low. Pressure at the exit of the compressor is high. The air would prefer to move from high to low (that would be a surge). The only thing that prevents it is mass flow of air.
At low rpm there is not much mass flow so the engine has a number of systems to ease the airs passage, primarily opening valves at the high pressure end to offload the pressure and varying the angle of the airflow at the low pressure end through inlet guide vanes and variable stator vanes.
At the design stage these were scheduled to keep the air moving and attached to the compressor blades but nothing is perfect at it would appear that the pressure gradient is still too high in this flight regime. Compressor stall is when a single stage of blades become turbulent, this would normally be at the high end, if this happens it can feed back and cause the stage in front to stall etc leading to a full surge (engine literally coughs it's gut's out the front).
By manually opening any bleed valve you are offloading the high end. Some engines will do this anyway, (PW1524 uses the engine anti ice valves to offload the compressor in lots of regimes) and therefore reducing the pressure gradient and increasing mass flow in the front of the compressor.