doing a Ladder test

something is going with that.^^
IMO it's more the speed of powder kernels coming out the muzzle exceeding the speed of sound, even from a suppressor.
I don't think they are being pushed from behind but being sucked from the front and they don't have the oxygen to burn properly until they hit the air.
 
That was the assumed answer. SEE is caused by collision of burning powder wave and slower muzzle gas wave.