Probus is correct in regards to startling quarry or prey. Where I live, pheasant and quail hunting are major (seasonal) activities. Because we have lot of wide open fields, it can be very difficult for an individual to flush out the birds - they'll simply run around behind you or off to one side while staying under the cover of the crops. They're smart enough to know that they have a better chance of surviving by evasion than by outright flight, because they're targets in the air. Not only is it much harder to see them close to the ground, it's also illegal and unethical to shoot flight birds on the ground.
It's common practice to get a group of people to walk side by side, spaced widely enough apart to flush a large portion of the field at once. This only allows the birds to move forwards towards the end of the field, and not slip around behind you. When the birds reach the end of the field, they'll then typically take to the air because they no longer have cover to hide in. Once they're airborne, the males can then be shot at.
The same practice applies to many kinds of game- drive it from cover, forcing it to expose itself, where it can be shot or captured. Even when you don't wish to kill or injure the animal, but merely capture for relocation, the same idea of flushing applies. If it's hidden in cover, you can't deal with it. Expose it, and you can take action.
In the case of combat, hidden combatants can shoot at you without you being able to shoot back. You then need to flush them out by some means.
In this case, the Marines would have been using an approach similar to my pheasant hunting - walking abreast spaced out at intervals to not only cover more ground, but reducing the likelihood of one attack hurting multiple Marines at once. They were trying to flush out the enemy, just like pheasants.
