I always chase the sergs since I know they are not going to dismount. I even run along side them without any fear of being attacked!
The few that dismount and attack me pay the price since the serg leaves them behind for me to feast on juicy bags!
I also stalk the few that do not have mount and are left behind; for some reason those are the tastiest bags since all it takes is a few hits and BANG a juicy and tasty bag goes to my inventory!
If you don't want to be left behind, learn to fight so that when someone comes and attacks you, you kill them and continue on your way.