When you start your engine on a calm day, standing still at the tarmac, What makes the air flow so easily through, say your CFM56?
The compressor rotor blades are small aerofoils, so they generate a lift force when the engine is rotating. They also generate what in a wing we would call downwash. The lift force acts in a forward direction, so the resulting downwash, acts in a rearward direction. This force the air to move rearwards through the compressor.
Air acts like current. It wants to balance inequal pressure and go from areas of higher pressure to areas of lower pressure until equal pressure exists. (taking the easiest way out)
You are correct in saying that air (and all other fluids) tends to flow from area of high pressure to areas of lower pressure. When the engine is running the pressure increases as the air flows from the front stages of the compressor to the rear stages. This suggests that the air will rend to flow forwards through the compressor.
We can see why this does not happen (most of the time) by imagining that we have taken a very thin slice of the compressor at right angles to the drive shaft. The slice is very thin so the static pressure (acting in a forward direction) on the rear face of the slice, will be equal and opposite to the static pressure (acting in a rearward direction) on the forward face of the slice. If the air is moving from front to rear we will also have a certain amount of dynamic pressure. But dynamic pressure acts only downstream in the direction of flow. So it will exert a rearward acting force on the front face of the slice, but no force on the rearward face. So we will have static pressure plus dynamic pressure acting rearwards and only static pressure acting forwards. So the air will tend to flow from the area of high total pressure at the front face, towards the lower total pressure at the rear face. As long as these conditions persist, the airflow will be from the front of the compressor to the rear.
In order to produce a flow reversal we need something to reverse the pressure difference across our slice of compressor. One such factor could be a sudden large increase in throttle setting. This would increase the fuel flow, which would in turn create an increase in static pressure in the combustion chamber. If this increase is sufficiently large, the (forward acting) static pressure in the combustion chamber will become greater than the (rearward acting) sum of static and dynamic pressure at the compressor outlet. This will cause the air to flow from the combustion chamber into the rear of the compressor. This reverse flow will then cause the blades of the rear stages of the compressor to stall, thereby permitting the reverse flow to continue to move forward. There are of course a number of other factors that can cause the airflow to reverse. Most of these involve aerodynamic stalling of the compressor blades (due to excessive angles of attack), but in all such cases the reverse flow is ultimately due to pressure differences.
So, once you start your engine and it achieves idle speed and is self sustainable, the pressure in the compressor is higher than ambient atmospheric pressure on the outside. What makes it not flow the easiest way out?
Is it the velocity of air coming in the duct?
It does always follow the easiest way out, but it may not be obvious that this is what it is doing.