in a random way, after you conquer an outpost, Pagan will send a few bad guys to try to conquer the outpost back.
It's unuseful to wait near the outpost, because it's random and you can wait a lot for nothing.
Since outpost are also quick travel point, once or another going away from the outpost you'll be advised to run back and kill all the bad guys.
Actually you don't have to kill them all, just a few (I never failed on this): after a certain number of foes in the dust the survivors will retire and after 3 times you'll unlock the achievo.
UPDATE (thanks to lxl Linc lxl)
This is also missable if you liberate the fortresses before getting 3
UPDATE 2 (thanks to xDeMiZEx86 )
If you conquered all the outpost and all the fortress, you can still reset them all even if you're in the post-game