this may not be the same flap but on the Kahr Q&A sight they say
http://www.kahr.com/OR-4.html#q1
Q. Why does the panel on the side of the grip of my Kahr polymer pistol sometimes "pop open"?
A. When a firearm is discharged, there is equal pressure on both sides of the ammunition, and the cartridge will seek the path of least resistance, which is forward. However, if the rim of the cartridge is cracked, the primer pinched, or any similar damage, pressure builds up and the path of least resistance is backwards causing the grip mount to pop. It is provided as a safety mechanism to prevent bodily injury.