First round pop can be described as a secondary dust/powder explosion in the suppressor. This can happen if the right combination of Oxygen level and powder gasses enter the suppressor at the right volume.  

In the event of First round pop, the second and later rounds will be quieter due to anaerobe conditions in the suppressor. Factors as caliber, powder type, gas pressure, volume in first chamber of the suppressor and atmospheric conditions plays a role in the event of first round pop. Normally cartridges with slower powder types and lower muzzle pressure is less inflicted by first round pop compared to faster powder types.