I just don't know how is the sound produced?
Does it come from the vibration of the blade of grass or the air in the up and down?
[Physics] Why can we produce a sound by blowing across a blade of grass, a paper strip or similar
acousticsfluid dynamicsresonanceturbulence
Related Solutions
Whilst physLads answer is correct, it is perhaps too succinct to be helpful with those unversed in gas dynamics, so I will attempt to explain what a 'blast wave' is in this case.
"My understanding currently is that the pockets created in the popper actually become high pressure pockets ( like a balloon) when moved very quickly through the air. This high pressure releases a small flap of paper suddenly when the differential pressure ( inside the pocket pressure is higher than what is just outside) is enough to push out the paper flap against frictional resistance."
This much is correct. However, as the flap of paper moves away, the high pressure gas pocket that had built up must rush in after it, pealing away layer by layer in a 'rarefaction' that means that, as the paper is moving it is followed by a moderately low pressure, but crucially very fast moving air. The paper is then brought to a very sudden halt (in the frame of reference of the persons hand), but still has a mass of fast moving air piling in towards it. This creates a 'blast wave', in this case a region of stationary high pressure air between the paper the region of fast moving low pressure air. The blast wave is the discontinuity in velocity and pressure between these two regions of air. As more air piles in across the blast wave the region of high pressure air adjacent to the paper increases in size, and the blast wave moves outwards through the air. This 'blast wave' or 'pressure discontinuity' is the loud pop heard.
Edit: you state in comments that you believe the sound to be produced by the region above the paper popper as the paper accelerates into it. It is true that this will also create a blast wave. However the acceleration of the paper is much slower than its deceleration, thus this wave will be smaller. In addition, the rarefaction caused when the paper halts will reduce the size of it yet further. Recordings of a paper popper being used may contain an ear splitting pop followed or preceeded by a much quieter one, the quieter one will be produced by this method.
Sound generated aerodynamically is a mature topic. There are some relevant lecture notes here: http://people.bath.ac.uk/ensmjc/Notes/tnoise.pdf (Although if you have access it's worth your while to read Lighthill's 1952 account).
In essence, correlations of Reynolds stresses (recall a stress is just a flux of momentum) due to regular fluctuations or turbulence in the flow can be shown to be equivalent to a distribution of acoustic quadrapoles, hence generating sound.
If you want a heuristic understanding then it's worth going through the thought experiment of sound generation due to a pulsating sphere. The velocity field of the fluid is related to the pressure through the Navier-Stokes equations. It can then be shown that fluctuations of the momentum imparted by the pulsating sphere to the fluid drives a pressure field and therefore sound is created.
Best Answer
Vortex shedding. Blowing on the sharp edge causes separation of the flow stream and the creation of vortices; rotating pockets of gas. The vortices are periodic and impart energy to the blade of grass causing it to vibrate.
Vortex shedding is problematic in aircraft as it steals lift from the wing. But it is a useful as a flow sensing (measurement) technology - and making buzzing sounds with grass blades!